Investigation of active sites using solid state 27Al and 31P MAS NMR in ceramic amorphous aluminophosphate materials prepared from different potassium salts of phosphate for the synthesis of diphenyl urea derivatives

Article abstract of DOI:10.1016/j.jpcs.2021.110087

Ceramic amorphous aluminophosphate (CAmAlP) catalysts were prepared by precipitation method using different phosphate salts of potassium such as KH₂PO₄, K₄P₂O₇ and K₂HPO₄ as well as H₃PO₄. The prepared materials were characterized by PXRD, FT-IR, XPS, SEM, BET Surface area, NH₃-TPD, ²⁷Al NMR and ³¹P NMR analytical methods. The catalytic activity of the materials was checked in the synthesis of diphenyl urea (DPU) from aniline and diethyl carbonate, under refluxing conditions. Further, the general application of the catalysts was tested using various substituted anilines. The recyclability of the catalysts was also studied. Uncertainties in percentage yields were calculated to check the reproducible surface properties. The P-XRD, BET Surface area and NH₃-TPD results indicated that the materials were amorphous with mesoporous texture, surface areas and acidities in the range 200–260 m²/g and 0.4–0.7 mmol/g respectively. ²⁷Al NMR studies revealed that Al is present in three different coordination states such as tetrahedral, pentagonal and octahedral. The relative percentages of these Al sites depends on the type of the potassium precursor phosphate salt used. Both tetrahedral and pentagonal Al sites in conjunction with each other represented catalytically active sites. An increase in the pentagonal sites contributed to additional increments to the catalytic activity of CAmAlP. The catalyst prepared from KH₂PO₄ was found to be the best and demonstrated 96% DPU yield.

Full text of DOI:10.1016/j.jpcs.2021.110087

Journal of Physics and Chemistry of Solids 154 (2021) 110087
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Journal of Physics and Chemistry of Solids
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Investigation of active sites using solid state 27Al and 31P MAS NMR in
ceramic amorphous aluminophosphate materials prepared from different
potassium salts of phosphate for the synthesis of diphenyl urea derivatives
,
*
N. Harisha, N. Kathyayinib, Bindhu Babyc, N. Nagarajua
a Catalysis Research Laboratory, Department of Chemistry, St. Joseph’s College PG and Research Centre, 36, Langford Road, Shantinagar, Bengaluru, 560 027,
Karnataka, India
b CIIRC, Jothy Institute of Technology, Tataguni, Off Kanakapura Road, Bengaluru, 560082, Karnataka, India
c Bishop Chulaparambil Memorial College (B.C.M), Kottayam, Kumily Road, Kottayam, 686001, Kerala, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Ceramic amorphous aluminophosphate (CAmAlP) catalysts were prepared by precipitation method using
different phosphate salts of potassium such as KH2PO4, K4P2O7 and K2HPO4 as well as H3PO4. The prepared
materials were characterized by PXRD, FT-IR, XPS, SEM, BET Surface area, NH3-TPD, 27Al NMR and 31P NMR
analytical methods. The catalytic activity of the materials was checked in the synthesis of diphenyl urea (DPU)
from aniline and diethyl carbonate, under refluxing conditions. Further, the general application of the catalysts
was tested using various substituted anilines. The recyclability of the catalysts was also studied. Uncertainties in
percentage yields were calculated to check the reproducible surface properties. The P-XRD, BET Surface area and
NH3-TPD results indicated that the materials were amorphous with mesoporous texture, surface areas and
acidities in the range 200–260 m2/g and 0.4–0.7 mmol/g respectively. 27Al NMR studies revealed that Al is
present in three different coordination states such as tetrahedral, pentagonal and octahedral. The relative per-
centages of these Al sites depends on the type of the potassium precursor phosphate salt used. Both tetrahedral
and pentagonal Al sites in conjunction with each other represented catalytically active sites. An increase in the
pentagonal sites contributed to additional increments to the catalytic activity of CAmAlP. The catalyst prepared
from KH2PO4 was found to be the best and demonstrated 96% DPU yield.
Ceramic amorphous aluminophosphates
27Al and 31P MAS NMR studies
Tetra and pentacoordinated sites
Acidity changes
Diphenyl ureas
1. Introduction
centers in ceramic crystalline metalaluminophosphate materials is
known at present to some extent of clarity, however, in the case of
Ceramic materials like crystalline aluminophosphates (ALPOs), sili-
coaluminophophates (SAPOs), metalalumimophosphates (MAPOs),
transition metal phosphates, phosphate ion impregnated metal oxides
and mixed metal oxides are well known for their catalytic activity in a
number of organic transformations [1–7]. Some of these materials have
a well-established long-range 3D network structure of the constituent
species, and a few show only short-range order, hence amorphous [8].
Synthesis of ceramic materials with ordered structure involves the use of
toxic organic templates whereas the amorphous materials are obtained
by straightforward and environmentally benign synthesis procedures.
One of the properties of these materials that needs to be addressed, in its
development for a particular synthetic application is the nature of the
catalytically active centers. Though the nature of catalytically active
ceramic amorphous metalphosphates, it is still a subject of debate and
challenge, because of the low reproducibility met in the preparation of
amorphous materials. Both theoretical and experimental methods have
been applied to evaluate the kind of active centers present in these
aluminophosphate materials. The former methods demand an ordered
structure of the material and hence application of such methods for
amorphous materials is not only limited but complicated. Experimental
methods seem to be more appropriate. Thus, by selecting appropriate
types of organic reactions and correlating the catalytic activity in these
reactions supported by their surface and bulk properties it is very
interesting to establish the nature of active sites.
Earlier, we have investigated the catalytic activity of ceramic
amorphous aluminophosphates (CAmAlP), transition metal phosphates
* Corresponding author.
E-mail address: nagarajun@yahoo.com (N. Nagaraju).
https://doi.org/10.1016/j.jpcs.2021.110087
Received 4 February 2021; Received in revised form 19 March 2021; Accepted 25 March 2021
Available online 31 March 2021
0022-3697/© 2021 Elsevier Ltd. All rights reserved.

N. Harish et al.
Journal of Physics and Chemistry of Solids 154 (2021) 110087
as well as those modified with different transition metal cations and
other anions [9,10]. We have reported the effect of several pre and
post-synthesis modifications, such as different sources of phosphate
ions, different pH conditions of precipitation, and various combinations
of metal ions on the catalytic activity of metal phosphates. In general,
the prepared aluminophosphates were found to be effective, economical
and eco-friendly heterogeneous catalytic materials for the synthesis of
diphenyl urea via a condensation reaction between aniline and diethyl
carbonate [11]. Further it was understood clearly that their catalytic
activity significantly depends on the synthesis modifications.
Table 1
Different phosphate sources, the amounts of Al and P salts used in the synthesis
of Ceramic amorphous Aluminophosphate catalysts.
Sl
Phosphate
source
Amount of
phosphate
source (g)
Amount of Al
(NO3)3⋅9H2O
(g)
Weight of
products
obtained
(g)
Catalyst
No
Abbreviation
1
2
3
4
H3PO4
4.9 (2.9 ml)
6.8
56.27
56.27
56.27
56.27
14.8
17.9
14.2
15.8
AlP(H3PO4)
AlP(KH2PO4)
AlP(K4P2O7)
AlP(K2HPO4)
KH2PO4
K4P2O7
K2HPO4
8.25
8.70
Researchers from references [12,13] have prepared CAmAlP using
different sodium and ammonium salts of phosphate and studied the ef-
fect of phosphate precursor on the catalytic activity and the nature of
active centers. In reference to these, we have investigated the nature of
active centers using 27Al and 31P MAS NMR studies in CAmAlP, prepared
by different potassium salts of phosphate as precursors.
700 ◦C, cooled and, stored in a desiccator until further use.
2.3. Characterization of catalysts
The synthesized materials were characterized for their surface and
bulk properties by the following analytical techniques. Powder X-ray
patterns were obtained by Pan AnalyticalX’pert pro X-ray diffractometer
The reasons for selecting different potassium salts for the preparation
of aluminium phosphates are as follows, it is well known from the
literature that the alkali metals (K & Na) have promoted the catalytic
activity of zirconium phosphate in the ethylene oxidation reaction [14,
15]. It was also demonstrated that the diameter of the crystallites has
more significant effect on the chemical properties than the structural
variation. The degree of coordination of surface atoms is a function of
crystallite sizes, the later could be altered with the different counter ions
of the precursor salt used for the preparation of the catalyst [16]. This
has prompted us to investigate the impact of size of the alkali metal
(here the larger potassium ion) on the catalytic activity of CAmAlP. The
catalytic activity of thus prepared Aluminophosphates was investigated
in the synthesis of DPU.
using Cu-K
α
radiation of λ = 0.154 nm in a 2θ range of 5◦ to 70◦ at 40 kV.
Infra-red spectra were recorded using Nicolet IR200 FT-IR spectropho-
tometer by KBr pellet technique. Surface acidity of the samples was
measured by TPD–NH3 desorption method on Mayura analytical in-
strument. The surface area, pore volume and pore diameter were
analyzed by Nova1000 Quanta Chrome gas sorption instrument. 27Al
and 31P solid-state MAS NMR were recorded on Bruker AV 700 (
υAl =
182.43 MHz) spectrometer which is equipped with a 16.4 T magnet
using a 2.5 mm probe head at a spinning frequency of 32 kHz. The
chemical shifts of 27Al and 31P NMR are referred to 1 M Al(NO3)3 and
H3PO4 respectively. All the quantitative 27Al MAS NMR spectra were
Diphenyl urea and its derivatives are important class of organic
compounds which are very useful in many biological applications as
fertilizers, antioxidants, resin precursors, medicines (breast cancer in-
hibitors and anti-tuberculosis agents) and also as intermediates for the
synthesis of carbamates [17–20]. DPU and its derivatives are commer-
cially synthesized by phosgene method which involves anilines and
phosgene (COCl2) which give rise to the isocyanate: a toxic biproduct
[21]. Diphenyl ureas were synthesized homogenously and hetero-
genously, the methods are throughly discussed in Ref. [22], involved the
elaborate reaction conditions, toxic reagents and environmentally un-
friendly processes. In realisation, with respect to the problems associ-
ated with the existing methods, right catalysts for the synthesis of
diphenyl ureas was very necessary and therefore we herewith made an
attempt to synthesize using CAmAlP prepared using different potassium
phosphate precursors.
recorded by direct polarization technique using a pulse width of 0.28μs
and 2000 scans were acquired for all the samples. The 31P MAS NMR
spectra were recorded using a pulse width of 1 μs and 32 scans were
acquired for all the samples. The experimental MAS NMR spectra were
fitted using the DMFIT [23] to find out the relative percentage popula-
tion of the different sites. XPS spectra were recorded using K-α™ + X-ray
photoelectron spectrometer (with C1s peak of binding energy at
284.9eV). Surface morphology of the samples were studied by the SEM
technique and was obtained from Ultra 55 instrument with the latest
Gemini technology. The powdered samples were spread on the carbon
tape, excess sample was blown off and further it was made conductive by
gold sputtering and was analyzed.
2.4. Catalytic activity studies
2. Experimental
The catalytic activity of the prepared aluminophosphates were tested
in the condensation reaction between aniline and diethyl carbonate to
yield diphenyl urea (Scheme 1).
2.1. Materials used
In a typical DPU synthesis reaction 10 ml of reaction mixture con-
taining 1:1 mol ratio of Aniline and diethyl carbonate were taken in a RB
flask attached with a water cooled condenser and containing 0.2 g of the
catalyst. The temperature of the RB flask was maintained between 130
and 135 ◦C. Initially within half an hour a solid product started forming
on the inner side walls of the RB flask. The reaction was left for
completion for two and half hours. The contents of the flask was treated
with hot ethanol to dissolve the organic solid product, then filtered to
separate the catalyst and the filtrate was subjected to rota evaporation to
isolate the solid product. The purity of the product was checked by TLC
and GC methods. Further the catalytic activity studies were conducted
using other substituted anilines to check the applicability of the catalyst
to synthesize other substituted DPUs.
Laboratory Grade Aluminium Nitrate [Al(NO3)3⋅9H2O] obtained
from LOBA Chemicals, H3PO4, KH2PO4, K2HPO4, K4P2O7, Liquor NH3,
aniline and diethyl carbonate were all obtained from SD fine chemicals
and are used as they were obtained from these commercial sources.
2.2. Preparation of ceramic amorphous aluminium phosphate
Calculated amounts of aluminium nitrate and potassium salt of
Phosphate/H3PO4 (Al:P:: 3:1), as indicated in Table 1, were mixed with
500 ml of hot water. The pH of the mixture was adjusted to 8 by drop-
wise addition of liquor ammonia solution. The obtained white precipi-
tate was kept aside undisturbed overnight. The precipitate was then
separated by filtration and washed with hot distilled water (80 ◦C). The
◦
precipitate was initially dried at 120 C for 10 h in a hot air oven to
3. Results and discussion
eliminate any physisorbed volatile impurities associated with it. The
amounts of the products obtained have been introduced in Table 1. The
material thus obtained was ground into a fine powder, calcined at
Initially the basic physicochemical characteristics of the prepared
catalysts such as, crystallinity, surface morphology, types of chemical
2

N. Harish et al.
Journal of Physics and Chemistry of Solids 154 (2021) 110087
Scheme 1. Condensation reaction between aniline and diethyl carbonate.
bonds, surface chemical composition, surface area and porosity, relevant
to catalytic materials are evaluated and the results are discussed below.
3.1. Powder XRD results
The XRD patterns (Fig. 1) of all the four prepared catalysts samples
were recorded in Bragg’s reflection 2θ range from 5◦ to 70◦. The cata-
lytic materials exhibited only a broad intense peak centered in a 2θ
range of 15◦ to 30◦.This indicated that the particles associated with the
catalysts do not possess any long-range order but are only microcrys-
talline in nature, i.e., they are basically amorphous materials [22].
Henceforth, in the following discussions at times the abbreviation
CAmAlP (Ceramic amorphous aluminophosphates) is used to represent
the catalysts.
3.2. Infrared spectroscopy results
From IR studies the presence of the characteristic chemical bonds
such as O–H, Al–O–H, P–O–H and P–O and P –O– P pertaining to the
prepared catalysts could be ascertained. Infrared spectra of the catalysts
are given in Fig-2. The broad peak between 3000 and 4000 cmꢀ 1 and a
low-intensity peak at 1640 cmꢀ 1 are due to the O–H bond stretching and
bending vibrations respectively of chemically adsorbed water molecules
[24]. The broad peak centered in between 1140 and 1110 cmꢀ 1 is
attributed to the stretching vibrations of tetrahedral P–O bonds [25].
This peak supports the presence of tetrahedral phosphorous associated
with tetrahedral aluminium which are interconnected by non-bridging
Fig. 2. Infrared spectra of samples prepared from different phos-
phate precursors.
oxygens. The broad peak in the range 700–980 cmꢀ 1 is attributed to a
mixture of both symmetric and asymmetric vibrations of P–O–P bonds
[26].
3.3. XPS studies
Surface composition of Al, P and O and their binding energies in the
prepared catalysts were determined by XPS. The XPS spectra of Al2p,
P2p3, P2p1 and O1s are shown in the fig-3. Table 2 depicts the quan-
titative results surface elemental composition with respect to their
specific binding energies.
The binding energies of Al,P and O were respectively found to be 75,
135 and 532eV which are in accordance with the values reported [26,
27]. The P peak positioned at 134–135 eV indicates phosphorous is
connected to O. The Binding energies between 528 and 536eV in O1s
spectra were attributed to the presence oxygen of alumina, free OH and
H2O [28]. The O1s graph also indicated that there are bridging, asym-
metric and non bridging oxygens. Bridging and non-bridging are found
at higher and lower binding energies respectively under the peak.
Asymmetrically bridging oxygens are found at intermediate binding
energy between bridging and non-bridging oxygens.
There is a slight shift of peak in the sample which is prepared from
KH2PO4 as phosphate source might be attributed to the slight increase in
the percentage of bridging oxygens. Non-bridging oxygens shown at
lower binding energies under the peak are specifically from POꢀ (shown
in IR graph at 1140-1110 cmꢀ 1) which combines with Al ions to form
Fig. 1. Powder XRD patterns of CAmAlP catalysts prepared from different
phosphate precursors.
3

N. Harish et al.
Journal of Physics and Chemistry of Solids 154 (2021) 110087
Fig. 3. XPS of CAmAlPs prepared from different phosphate precursors (a) Al2p scans (b) P2p1 scans (down ones on each curve) and P2p3 scans (top ones on each
curve) (c) O1s scans.
solubility products of aluminium hydroxide and aluminium phosphate,
Table 2
and the former being lower than the later, Al(OH)3 get precipitated
XPS Peak positions and fractions of Al, P and O in XPS spectra of all the samples
initially and then AlPO4.
prepared by different phosphate sources.
Catalyst (source)
O-1s
Al-2p
P-2p
3.5. BET surface area results
AlP(H3PO4)
%
62.5
28.5
74.8
26.4
75.0
27.3
74.7
27.0
75
9.0
Binding energy
531.9
63.5
134.0
10.0
134.0
10.2
134.0
10.5
134.0
Surface area, pore volume and pore diameter of the CAmAlPs were
obtained by nitrogen adsorption-desorption studies. The BJH isotherms
of nitrogen adsorption-desorption are given in fig-5. In these isotherms
the hysteresis loops appeared at higher partial pressures (P/Po > 0.6)
and hence represent type IV isotherms [29]. and this is attributed to an
intrinsic mesoporosity in the samples [30]. The surface areas of the
samples were in the range 200–300 m2/g (Table 3), which is neither
high nor too low for a catalytic material. When the surface area values of
the catalysts prepared using different phosphate sources, were
compared with each other, very little variation was noticed. However,
the catalyst prepared using KH2PO4 as the phosphate source had the
highest surface area. It is a known fact that as the surface area increases
the number of active sites on the surface also will increase. The surface
areas of the prepared catalysts were found to be in the increasing order
AlP(KH2PO4)
AlP(K4P2O7)
AlP(K2HPO4)
%
Binding energy
532.0
62.5
%
Binding energy
%
531.8
62.5
Binding energy
532.0
tetrahedral Al(OP)4 species which is proved by NMR studies [26].
3.4. Scanning electron micrographs (SEM)
The SEM images (Fig-4) showed the presence of particles with het-
erogeneous sizes and shapes and hence no particular morphology could
be assigned. The heterogeneity is attributed to the presence of an inti-
mate composite of AlPO4 and alumina [22]. Due to the difference in the
4

N. Harish et al.
Journal of Physics and Chemistry of Solids 154 (2021) 110087
Fig. 4. Scanning electron micrographs of Ceramic aluminophosphates prepared from different phosphate precursors.
AlP(KH2PO4) >AlP(H3PO4) > AlP(K2HPO4) >AlP(K4P2O7)
concentration of pentacoordinated Al species as well as the associated
tetrahedral phosphate groups.
The pore volumes of the catalysts were in the range 0.7–1.0 cm3/g
and was found to increase in the same order as that of their surface area.
3.6. TPD ammonia results
3.7. 27Al and 31P MAS NMR results
TPD ammonia studies were carried out to evaluate the acidic
strength and acid sites concentration of the prepared catalysts. The
quantity of ammonia gas adsorbed gives the total acidity. TPD profiles
indicating the quantity of ammonia desorbed from the surface of the
In a pursuit to find the relationship between the chemical environ-
ment of the Al and P sites in aluminophosphates prepared from various
potassium phosphate precursors, and their catalytic activity, all the
prepared samples were characterized by recording 27Al and 31P MAS
NMR spectra (Fig. 7a and b).
◦
◦
catalysts in the temperature range from 50 C to 700 C are given in
Fig. 6. The striking feature of all these curves is that most of the adsorbed
ammonia is desorbed in the lower temperature range i.e. 100 – 250 ◦C,
indicating the presence of only acid sites with moderate strength [31].
The acid sites concentration was calculated from the area under the
desorption peak and were found to be in the range 0.30–0.65 mmol/g
(table- 3). The acidity of these catalysts may be attributed to the protons
associated with the surface – OH groups bonded to Al and P atoms
(bronsted acidity) as well as to the incompletely coordinated Al species
(Lewis acidity) [32]. Very subtle difference in the acid sites concentra-
tion was observed among the four catalysts and the variation of their
acidities was in the following increasing order, which is the same as that
of their surface area. AlP(KH2PO4) >AlP(H3PO4) > AlP(K2HPO4) >AlP
(K4P2O7).
27Al MAS NMR spectra showed 3 different distinct peaks with the
chemical shift values in the range 75 to ꢀ 25 ppm. The chemical shift
values representing different types of Al species and their relative per-
centages are shown in Table 4.
The coordination of Al pertaining to each of these peaks was assigned
as per the related literature reported [33–37]. Accordingly, the peak
between 65 and 40 ppm represents tetrahedral AlO4 units (Td-AlO4) of
alumina. Alumina is obtained from the precipitated aluminum hydrox-
ide along with aluminium phosphate during its preparation. The peak in
the range 35 to 25 ppm is due to the tetrahedrally coordinated Al(OP)4
species. The peaks in between 15 and ꢀ 10 ppm represent penta and hexa
coordinated Al sites. A few references [34,38] states that the chemical
shift values of Hexa coordination are lower than the pentagonal coor-
dination peaks in aluminophosphates. The composition of these penta
and hexa coordinated species is Al(OP)x (OH)y.
The difference in the total acidity of the catalysts may be attributed
to the difference in the nature of the degree of different types of coor-
dination of Al. The higher acidity of the catalyst prepared from KH2PO4
is thus due to the presence of tetrahedral alumina and the higher
The peak representing Td-AlO4 was found to be dominant in all the
four AlP catalysts, as observed by the area under the peak. The presence
5

N. Harish et al.
Journal of Physics and Chemistry of Solids 154 (2021) 110087
(KH2PO4)}, associated with pentacoordinated Al species is higher than
those {AlP(K4P2O7) and AlP(K2HPO4)} with octahedral species.
31P NMR spectra of all the samples exhibited a broad peak with the
chemical shift values in the range ꢀ 5 to ꢀ 35 ppm attributed to tetra-
hedral phosphate species [45]. According to the reported literature [39,
46,47] chemical shift values in this range represent three different
tetrahedral phosphate species, such as (a) phosphate species attached to
AlO6 (octahedral) units in the range ꢀ 7.8 to ꢀ 16.2 ppm (b) phosphate
species coordinated to AlPO5(pentagonal) units in the range ꢀ 8.2 to
ꢀ 15.7 ppm and (c) phosphate groups associated with AlO4 (tetrahedral)
units in the range ꢀ 11.5–5.9 ppm. All the mentioned chemical shifts are
noticed in the 31P NMR spectra of the prepared catalysts, but covered
under a broad peak (Fig. 7b). However, due to the poor resolution of the
peaks, they were not deconvoluted, as it may lead to wrong
interpretation.
The following points may be further be noted from the results pre-
sented in Table 4:
a. Aluminium in the catalyst is present in three differently coordinated
environments – tetrahedral, pentagonal and octahedral.
b. Tetrahedrally coordinated Al sites are present in all the samples, as
AlO4 and Al(OP)4 units irrespective of the nature of the potassium
precursor salt used for their preparation. The relative percentage of
AlO4 and Al(OP)4 units varies with the nature of the precursor.
1. AlO4 increases in the order K2HPO4 = H3PO4 > KH2PO4 > K4P2O7
2. AlOP4 increases in the order K4P2O7>K2HPO4 > H3PO4 > KH2PO4
c. The hexa and penta coordinated units have composition Al(OP)x
(OH)y. The octahedral units are formed only when P2O47ꢀ and HPO4ꢀ 2
Fig. 5. BJH isotherms for the samples of Ceramic Aluminophosphates prepared
from different phosphate precursors.
and coexistence of 3 differently coordinated Al species was further
confirmed using MQMAS (3QMAS) NMR studies, the detailed explana-
tion is reported in our earlier work [13]. It is said that alumina (Td-AlO4)
is present in only tetrahedral coordination [39], however in our samples
the addition of hydroxyl ions resulted in the penta and hexa coordinated
Al sites which had unbalanced charges and these along with Td-AlO4
represent the potential catalytically active sites on the surface of
CAmAlPs the materials. These results further confirm that a mixture of
alumina and aluminophosphate is formed during precipitation.
Td-Al(OP)4 is formed by Al which is tetrahedrally coordinated to
phosphate. This species is charge neutral and non-acidic. It is found that
the catalyst prepared from KH2PO4 as phosphate precursor had lowest
percentage of these species. This inturn has increased the concentration
of pentacoordinated Al species.
Penta coordinated Al species could be in distorted trigonal bipyra-
midal geometry with D3h symmetry [40,41]. The axial bonds of penta-
coordinated Al species are less stable and hence energetically potential
active sites for catalytic activity [42–44]. These Pentacoordinated Al
sites were observed in AlP catalysts prepared from H3PO4 and KH2PO4 as
phosphate sources and their percentage was found to be more in the
later case (29%) than in the former (20%). The more stable octahedral Al
species are formed only in the catalysts prepared from K4P2O7 and
K2HPO4 as phosphate sources with 22 and 14% respectively. Further, is
to be noted that NH3 -TPD studies, as discussed later in the text, also
indicated the surface acidity of the catalysts {AlP(H3PO4) and AlP
Fig. 6. Ammonia Desorption curves of all the samples prepared from different
phosphate precursors.
Table 3
Surface properties of Ceramic amorphous aluminophosphate materials prepared from different phosphate sources.
Sl no
Catalyst (Phosphate source)
Surface area m2/g
Total pore volume (cc/g)
Average pore diameter (Å)
No of active sites
Acidity
Nature
mmol/g
1
2
3
4
AlP(H3PO4)
AlP(KH2PO4)
AlP(K4P2O7)
AlP(K2HPO4)
252
267
210
237
0.94
0.96
0.70
0.79
106.5
105.8
110.4
108.6
2.52 × 1013
2.67 × 1013
2.10 × 1013
2.37 × 1013
0.58
0.63
0.38
0.42
Amorphous
Amorphous
Amorphous
Amorphous
6

N. Harish et al.
Journal of Physics and Chemistry of Solids 154 (2021) 110087
Fig. 7. (a) 27Al NMR and (b) 31P NMR Spectra of aluminophosphates samples prepared from different potassium phosphate precursors.
Table 4
27Al and31P MAS NMR results of catalysts prepared from different phosphate precursors.
Sl NO
Catalyst
27Al NMR
31P NMR
Td-AlO4
Td-Al(OP)4
penta coordinated
Al(OP)x (OH)y
Hexa coordinated
Al(OP)x (OH)y
Td-PO4
∂
iso
Td
%
∂
iso
Td
%
∂
iso
penta %
∂
iso
octa %
∂
iso
Td %
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
PO4
1
2
3
4
AlP(H3PO4)
AlP(KH2PO4)
AlP(K4P2O7)
AlP(K2HPO4)
58.7
56.6
56.8
57.3
59.5
54.0
49.6
61.5
30.7
29.8
28.3
29.2
20.5
17.0
28.4
24.8
3.2
3.3
20.0
29.0
ꢀ 20.68
ꢀ 20.66
ꢀ 19.50
ꢀ 19.77
100
100
100
100
ꢀ 3.7
ꢀ 0.7
22.0
14.0
were used as the precursor salts and pentagonal sites with H3PO4
and H2POꢀ4 1
>
Table 5
Mole percentage yields and selectivities for condensation reaction between an-
iline and dimethyl carbonate.
d. Phosphorous in the catalyst is only tetrahedrally coordinated
possibly with only oxygen atoms which are in turn are coordinated Al
sites.
Sl
Catalyst
Condensation reaction between Aniline and Diethyl
carbonate
NO
e. TPD-NH3 surface acidity measurement studies revealed the fact that
the catalyst obtained from KH2PO4 precursor had higher acidity than
the other catalyst, which is attributed to the presence of pentagonal
Al species.
mol % Aniline
conversion
mol %
DPU
mol % of other
products
1
2
AlP(H3PO4)
AlP
100
98
90
96
10
2
(KH2PO4)
AlP
Both Td and pentagonal Al sites in conjunction with each other
represent catalytically active sites however an increase in the pentag-
onal sites contribute to an additional increment to the catalytic activity
of CAmAlPs.
3
4
90
58
86
32
14
(K4P2O7)
AlP
100
(K2HPO4)
3.8. Catalytic activity studies
for the preparation of CAmAlPs catalysts. The difference in the per-
centage yields of the main, as well as the byproducts further indicated
that the concentration and strength and of the surface acidic active sites
are different. This difference in the active sites is mainly due to the
variation in the precursor potassium salts used in the preparation of the
catalytic CAmAlPs.
The catalytic activity of the CAmAlPs prepared in this work was
evaluated in an acid-catalyzed reaction i.e. between aniline and
dimethyl carbonate, under refluxing conditions. The % conversion of
aniline, % yield of Diphenyl urea and the other products formed are
presented in Table 5. It is noticed at the outset that all the catalysts
showed activity in catalyzing the organic reaction. Further, when the
reaction was conducted without any catalyst or a solid base, such as
CaO, it did not yield any expected product. The fact that the reaction did
occur in the presence of all the CAmAlPs, their acidic nature is
confirmed, as determined by NH3-TPD studies. The % conversions of
aniline was almost 100% and the % yield of the expected major product
of the reaction i.e., Diphenyl urea (DPU) was in the range of 58–96%.
The purity of the product though was found to be good, the % of
byproducts formed varied with the type of precursor potassium salt used
The yield of the product has a close relevance with the number of
catalytically active acid sites, which in turn related to the surface area of
the catalysts, whereas the purity of the product or the formation of
byproducts depend on the nature and strength of the acid sites (Table 3).
In the present investigation as described under the characterization
sections that the surface area and acid sites concentrations of CAmAlPs
are in the following increasing order; AlP(KH2PO4) >AlP(H3PO4) > AlP
(K2HPO4) >AlP(K4P2O7), and the catalytic activity also vary in the same
order. This order of catalytic activity has a good correlation with the
7

N. Harish et al.
Journal of Physics and Chemistry of Solids 154 (2021) 110087
total number of tetrahedral (Td-AlO4) and pentagonal Al species. It may
be stated that the Brønsted and Lewis acid sites associated with these Al
sites are the potential catalytic sites in the prepared CAmAlPs. The
catalysts, AlP(KH2PO4) and AlP(H3PO4) which exhibited higher per-
centage of these Al sites also showed higher catalytic activity. Further it
is also to be noted that as Td-Al(OP)4 increases the total conversion to
DPU decreases. This is due to its charge neutral non-acidic nature. Thus
the catalyst AlP(KH2PO4) which has less amount of Td-Al(OP)4 showed
higher conversion to DPU. The presence of these Td-AlO4, Td-Al(OP)4
and pentagonal Al sites are confirmed by 27Al MAS-NMR studies.
Sodium and ammonium salts of phosphates and H3PO4 as phosphate
source in the synthesis of CAmAlPs were studied recently [13] and re-
ported that the catalyst prepared from H3PO4 showed better catalytic
activity towards the synthesis of DPU, than the ones prepared from so-
dium ammonium sources of phosphates. In the present studies, where
potassium salts are used in the preparation of catalysts, higher yields and
selectivity of DPU were noticed. This is attributed to the generation of
larger number of active sites due to higher surface areas and pore vol-
umes of the catalysts. Further the catalyst which is prepared from
KH2PO4 as phosphate source was found to be the best. The higher sur-
face areas and pore volume of the catalyst may be due to the following
reasons. The catalysts prepared from sodium sources of phosphates
showed the presence of sodium on their surfaces which would block the
pores. In the present catalysts where potassium phosphates as sources
were devoid of potassium on the surface which has led to the higher
surface areas and pore volumes and therefore the higher catalytic ac-
tivities compared to catalysts where sodium sources used. The higher
acidity is due to presence of higher percentage of pentacoordinated Al
species. Al NMR studies showed the catalyst prepared from KH2PO4 as
phosphate source showed highest percentage of pentacoordinated Al
sites, which in turn showed higher acidity and hence catalytic activity as
compared to sodium sources.
As indicated by PXRD analysis, the prepared catalysts are amorphous
and are associated with inherent heterogeneity in composition and
structure due to the lack of long range order. In such materials, it is
difficult to maintain the same surface properties when prepared at
different batches, as their surface properties are sensitive to the method
and conditions of synthesis. We have made an attempt to see this effect
on the catalytic activity of CAmAlPs. Hence, three sets of the same
catalysts were prepared, at different times maintaining the same syn-
thesis conditions every time. We have checked the Catalytic activities of
thus prepared materials were determined in the synthesis of DPU. The
uncertainties in the percentage yield obtained for each set of catalysts
were calculated using the following equations [48]. The results are
presented in Table 7. The calculated in the percentage yield were used to
evaluate whether the surface properties of the catalysts are reproducible
or not.
√̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅
(
√
√
√
√
ꢀ
)
∑
∑
2
N
N
1
xi ꢀ N
i=1xi
i=1
S =
N ꢀ 1
Equation 1: formula to calculate standard deviation
S = standard deviation, xi = value of the measurement, N = number
∑
N
of degrees of freedom (number of replicate measurements) and i=1Xi.
= sum of the degrees of freedom (sum of replicate measurements).
The Confidence interval was calculated using the formula.
ts
N
√̅̅̅̅
u = X ±
Equation 2: formula to calculate uncertainty error
Where
μ is the confidence interval, x = mean of replicate measure-
ments N= Number of replicate measurements t = value for confidence
level probability (at 95% class interval the t value was found to be 4.30)
The formulas were reproduced from reference [48] The uncertainties of
percentage yields are mentioned in detail in the table no 7.
Further the catalyst obtained from K4P2O7 as phosphate source, was
devoid of pentacoordinated Al sites, showed least catalytic activity
similar to catalysts prepared from sodium sources. These comparative
studies helped us to prove that the catalysts prepared from KH2PO4 as
phosphate source was the best catalyst for the preparation of DPU
compared to all other sodium, ammonium and potassium sources. The
general application of AlP(KH2PO4) as a catalyst for the preparation of
other substituted DPUs was investigated, by conducting reactions under
the same conditions as that between aniline and diethyl carbonate, using
differently substituted anilines and diethyl carbonate. The results are
presented in Table – 6.
The uncertainties were high for the catalyst which was prepared by
K4P2O7 as phosphate source and we cannot say it is highly not a suitable
catalyst for the above mentioned reaction. We did not obtain the
preferred products due to the difference in selectivity and all along there
is a much larger difference in uncertainty compared to others which
make it less suitable. Lesser uncertainty results were obtained from the
catalysts AlP(H3PO4) and (AlP)KH2PO4 making them more selective
with good yields. These lesser uncertainty values says this catalyst is best
suitable for above mentioned reaction. This is also best attributed to
beneficial reproducible surface properties compared to the catalysts
(AlP)K4P2O7 and (AlP)K2HPO4.
The selected catalyst showed good catalytic activity and selectivity
towards the formation of other urea derivatives. In general, it was
observed that the presence of electron donating groups on the aromatic
ring gave rise to higher yields of the expected product compared to
electron withdrawing groups which are present. Presence of electron
donating groups on the aromatic ring of aniline makes the nitrogen atom
electron rich and can attack partially positive carbonyl carbon in the
carbonate readily. Particularly electron donating group at para position
give rise to high conversion and selectivity towards Diphenyl Ureas.
Presence of electron withdrawing group like –NO2 on anilines makes N
of –NH2 as slightly electron deficient to attack partially positive
carbonyl carbon thereby decreasing the yield of DPU derivative. In
addition to these electronic effects, position of substituents on the aro-
matic ring also has also played a significant role towards the percentage
yield of ureas. When compared to reactants with substituents at ortho
positions, reactants with substituents at para positions gave higher yield
and selectivity towards ureas. Substituents at ortho positions on aro-
matic rings are sterically repelled by the lone pair of electrons from the
nitrogen atom, resulting in a decrease in yield. Substituents at para
position experienced lesser steric hindrance compared to the sub-
stituents at ortho positions. As a result, the reactants p-toludine, p-
chloroaniline, and p-nitroaniline provided the highest yields and selec-
tivities for their respective ureas.
4. Conclusions
Aluminophosphate samples prepared from different phosphate salts
of potassium were found, from 27Al and 31P MAS NMR studies, to be
associated with three types i.e. Tetrahedral, pentagonal and octahedral
Al sites. The relative concentrations of these sites depend on the type of
phosphate salt of potassium. Thus 27Al NMR investigations of CAmAlP
supplemented in the correlation of catalytic activity and the nature of
coordination of Al species in the catalysts. The materials were found to
be good catalysts in the reaction between aniline (and its derivatives)
and dimethyl carbonate in the synthesis of diphenyl urea, with up to
96% yield of the expected product. Pentagonal Al sites in conjunction
with Td sites showed a beneficial effect on catalytic activity. Catalysts
prepared from KH2PO4 as phosphate sources showed better catalytic
activities compared to other phosphates sources used. The uncertainties
in the percentage yield of the product were minimum.
Funding
This research did not receive any specific grant from funding
8

N. Harish et al.
Journal of Physics and Chemistry of Solids 154 (2021) 110087
Table 6
Generality studies for Condensation reaction with different anilines using CAmAlPs catalysts prepared using KH2PO4 as phosphate source.
Sl NO
Amines
% of amine conversion
Products (Ureas)
mol % yield of Ureas
mol % of other products
2
1
98
96
1,3-di-phenylurea
Aniline
2
3
100
88
100
71
0
1,3-di-p-tolylurea
1,3-di-m-tolylurea
p-Toludine
17
m-Toludine
o-Toludine
4
5
80
91
57
72
23
19
1,3-di-o-tolylurea
1,3-bis(4-methoxyphenyl)urea
p-Anisidine
6
7
79
92
64
82
15
10
o-Phenylenediamine
1,3-bis(2-aminophenyl)urea
1,3-bis(4-chlorophenyl)urea
p-Chloroaniline
8
73
62
11
1,3-bis(3-chlorophenyl)urea
m-Chloroaniline
o-Chloroaniline
9
62
70
56
54
8
1,3-bis(2-chlorophenyl)urea
1,3-bis(4-nitrophenyl)urea
10
16
p-Nitroaniline
m-Nitroaniline
11
81
68
13
1,3-bis(3-nitrophenyl)urea
(continued on next page)
9

N. Harish et al.
Journal of Physics and Chemistry of Solids 154 (2021) 110087
Table 6 (continued)
Sl NO
Amines
% of amine conversion
58
Products (Ureas)
mol % yield of Ureas
42
mol % of other products
16
12
o-Nitroaniline
1,3-bis(2-nitrophenyl)urea
[6] A.I. Prymak, O. Prymak, J. Wang, V. Narayana, A. Martin, U. Bentrup, I. Prymak,
O. Prymak, J. Wang, N. Kalevaru, A. Martin, Phosphate functionalization of CeO2
-ZrO2 solid solutions for the catalytic formation of dimethyl carbonate from
methanol and carbon dioxide, ChemCatChem 10 (2018) 391–394, https://doi.org/
10.1002/cctc.201701105.
Table 7
The estimated uncertainties in the product yield from three different sets of each
catalyst prepared in the Condensation reaction between aniline and diethyl
carbonate. (A,B and C represent three different sets of catalysts prepared and the
respective mol % yields of the product DPU obtained).
[7] K.B. Ghoreishi, N. Asim, M.A. Yarmo, M.W. Samsudin, Mesoporous phosphated and
sulphated silica as solid acid catalysts for glycerol acetylation, Chem. Pap. 68
(2014) 1194–1204, https://doi.org/10.2478/s11696-014-0550-x.
[8] B.R. Goldsmith, B. Peters, J.K. Johnson, B.C. Gates, S.L. Scott, Beyond ordered
materials: understanding catalytic sites on amorphous solids, ACS Catal. 7 (11)
(2017) 7543–7557, https://doi.org/10.1021/acscatal.7b01767.
Sl.
Catalysts
A
B
C
Avg mol %
DPU
Uncertainty
estimate
No
1
2
AlP(H3PO4)
AlP
90
96
90
97
89
96
89.66
96.33
89.66 ± 1.43
96.33 ± 1.43
[9] A. Hamza, N. Nagaraju, Amorphous metal - aluminophosphate catalysts for aldol
condensation of n - heptanal and benzaldehyde to jasminaldehyde, Chin. J. Catal.
36 (2015) 209–215, https://doi.org/10.1016/S1872-2067(14)60206-0.
[10] A. V Vijayasankar, N. Nagaraju, Preparation and characterisation of amorphous
mesoporous aluminophosphate and metal aluminophosphate as an efficient
heterogeneous catalyst for transesterification reaction, Compt. Rendus Chem. 14
(2011) 1109–1116, https://doi.org/10.1016/j.crci.2011.09.013.
(KH2PO4)
AlP(K4P2O7)
AlP
3
4
58
86
55
84
64
83
58.66
84.33
59 ± 11.37
84.33 ± 3.79
(K2HPO4)
[11] M. Rekha, N. Nagaraju, Effect of anion on the catalytic activity of cobalt
aluminophosphate in the synthesis of N, N-biphenyl urea derivatives, advances in
inorganic phosphate materials, ceramic transactions, in: Ilias Belharouak, Vilas. G.
Pol, Wiley, 2012, pp. 159–168, https://doi.org/10.1002/9781118491638.
12 M. Campello, M. Jaraba, D. Luna, R. Luque, J.M. Marinas, A.A. Romero, Effect of
phosphate precursor and organic additives on the structural and catalytic properties
of amorphous mesoporous AlPO4 materials, Chem. Mater. 15 (2003) 3352–3364,
https://doi.org/10.1021/cm030206+.
agencies in the public, commercial, or not-for-profit sectors.
Credit author statement
Mr Harish N : Conceptualization, Methodology, Software, Investi-
gation, writing- original draft preparation, Reviewing and editing ,
formal analysis.
[13] N. Harish, N. Kathyayini, B. Bindhu, N. Nagaraju, Investigation on the nature of
catalytically active sites in non-crystalline aluminophosphates from 27Al and 31
MAS-NMR studies, J. Non-Cryst. Solids 522 (2019) 119577, https://doi.org/
10.1016/j.jnoncrysol.2019.119577.
P
Dr. H. Kathyayini : Visualization, Resources, supervision.
Dr. Bindhu Baby : software, Data Curation and Validation.
Dr.N.Nagaraju : Supervision, project administration, Reviewing and
editing.
[14] S. Cheng, A. Clearfield, Oxidation of ethylene catalyzed by silver supported on
zirconium Phosphate : particle size and support effect, J. Catal. 467 (1985)
455–467, https://doi.org/10.1016/0021-9517(85)90210-6.
[15] A. Clearfield, D.S. Thakur, Zirconium and titanium phosphates as catalysts: a
review, Appl. Catal. 26 (1986) 1–26, https://doi.org/10.1016/S0166-9834(00)
82538-5.
[16] K. Chaitra, R.T. Vinny, P. Sivaraman, N. Reddy, C. Hu, K. Venkatesh, C.S. Vivek,
N. Nagaraju, N. Kathyayini, KOH activated carbon derived from biomass-banana
fiber s as an efficient negative electrode in high performance asymmetric, J. Ener.
Chem 26 (1) (2017) 56–62, https://doi.org/10.1016/j.jechem.2016.07.003.
[17] W.A. Skinner, H.T. Crawford, L.C. Rutledge, M.A. Moussa, Topical mosquito
repellents XII: N-substituted ureas and cyclic ureas, J. Pharmacol. Sci. 68 (3)
(1979) 391–392, https://doi.org/10.1002/jps.2600680342.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
[18] T.P. Vyshnyakova, I.A. Golubeva, E. V Glebova, Substituted ureas, synthetic
methods, and applications, Russain Chemical. Reviews (Engl. Transl.) 54 (1985)
249–261, https://doi.org/10.1070/RC1985v054n03ABEH003022.
[19] Y. Zhan, H. Zhang, J. Li, Y. Zhang, J. Zhang, L. He, A novel biphenyl urea derivate
inhibits the invasion of breast cancer through the modulation of CXCR 4, J. cellu.
and mol. Medi. 19 (7) (2015) 1614–1623, https://doi.org/10.1111/jcmm.12536.
[20] J.R. Brown, E.J. North, J.G. Hurdle, C. Morisseau, J.S. Scarborough, D. Sun,
J. Kordulakova, M. S Scherman, V. Jones, A. Grzegorzewicz, R.M. Crew the
structure–activity relationship of urea derivatives as anti-tuberculosis agents,
Bioorg. Med. Chem. 19 (18) (2011) 5585–5595, https://doi.org/10.1016/j.
bmc.2011.07.034.
Acknowledgements
The authors thank St. Joseph’s college for XRD analysis and other
facilities, Dr. T.G. Ajith Kumar, Central NMR Facility, CSIR - NCL Pune.
The authors also express gratitude to CIIRC JIT for TPD and GC analysis,
BIT Bengaluru for BET Surface area Measurements.
[21] P. Majer, R.S. Randad, A safe and efficient method for preparation of N, N’-
unsymmetrically disubstituted ureas utilizing triphosgene, J. Org. Chem. 59 (7)
(1994) 1937–1938, https://doi.org/10.1021/jo00086a061.
References
[1] H. Kim, J. Park, I. Park, K. Jin, S.E. Jerng, S.H. Kim, K.T. Nam, K. Kang,
Coordination tuning of cobalt phosphates towards efficient water oxidation
catalyst, Nat. Commun. 6 (2015) 1–11, https://doi.org/10.1038/ncomms9253.
[2] S.M. Patel, V. Chudasama, P.A. Ganeshpure, Metal ( IV ) phosphates as solid acid
catalysts for selective cyclodehydration of 1 , n –diols, Green Chem. (2001)
143–145, https://doi.org/10.1039/b100503k.
[22] N. Harish, N. Kathyayini, N. Nagaraju, Studies on the catalytic activity of
mesoporous alumina-aluminophosphate (Al2O3-AlPO4) materials in the synthesis
of N,N’-Diphenyl urea, React. Kinet. Mech. Catal. 125 (2018), https://doi.org/
10.1007/s11144-018-1457-2.
[23] D. Massiot, F. Fayon, M. Capron, I. King, L. Calv, B. Alonso, J. Durand, B. Bujoli,
Z. Gan, G. Hoatson, Modelling one- and two-dimensional solid-state NMR, Magn.
Reson. Chem. 40 (2002) 70–76, https://doi.org/10.1002/mrc.984.
[24] G. Kuriakose, J.B. Nagy, N. Nagaraju, Structure and catalytic activity correlation of
CoAlPO4 in the synthesis of N , N -biphenyl urea via alkylation of aniline using
dimethyl carbonate, Catal. Commun. 6 (2005) 29–35, https://doi.org/10.1016/j.
catcom.2004.10.006.
´
[3] R. Weingarten, Y. Tae, G.A. Tompsett, A. Fernandez, K. Sung, E.W. Hagaman,
W. Curt, C. Jr, J.A. Dumesic, G.W. Huber, Conversion of glucose into levulinic acid
with solid metal(IV)phosphate catalysts, J. Catal. 304 (2013) 123–134, https://doi.
org/10.1016/j.jcat.2013.03.023.
[4] R. Thakkar, U. Chudasama, Synthesis of mono and diesters using eco-friendly solid
acid catalyst zirconium titanium phosphate, Green Chem. Lett. Rev. 2 (2009)
37–41, https://doi.org/10.1080/17518250903039774a.
[25] T. Roncal-herrero, J.D. Rodrı, L.G. Benning, E.H. Oelkers, Precipitation of iron and
aluminum phosphates directly from aqueous solution as a function of temperature
from 50 to 200 ◦ C, Cryst. Growth Des. (2009) 45–48, https://doi.org/10.1021/
cg900654m.
[5] A. Clearfield, M. Xiao, Y. Meng, L. Sun, Solid acid catalyst based on single-layer
α
-zirconium phosphate nanosheets for biodiesel production via esterification,
Catalysts 8 (2018) 17–24, https://doi.org/10.3390/catal8010017.
10

N. Harish et al.
Journal of Physics and Chemistry of Solids 154 (2021) 110087
[26] S.W. Yung, H.Y. Chiang, Y.S. Lai, F.B. Wu, C. Fu, Y. Lee, Thermal , optical and
structural properties of Tb doped zinc aluminum phosphate glasses, Ceram. Int. 41
(2014) 1–12, https://doi.org/10.1016/j.ceramint.2014.09.005.
[38] R. Youngman, NMR spectroscopy in glass science: a review of the elements,
Materials (Basel). 11 (4) (2018) 476–500, https://doi.org/10.3390/ma11040476.
[39] S. Wegner, L. van Wüllen, G. Tricot, The structure of aluminophosphate glasses
revisited: application of modern solid state NMR strategies to determine structural
motifs on intermediate length scales, J. Non-Cryst. Solids 354 (2008) 1703–1714,
https://doi.org/10.1016/j.jnoncrysol.2007.10.034.
[27] J.A. Rotole, P.M.A. Sherwood, Aluminum phosphate by XPS, Surf. Sci. Spectra 5
(1998) 60–66, https://doi.org/10.1116/1.1247858.
[28] N. Kumar, K. Biswas, Cryomilling : an environment friendly approach of
preparation large quantity ultra refined pure aluminium nanoparticles, Integr.
Med. Res. 8 (2017) 63–74, https://doi.org/10.1016/j.jmrt.2017.05.017.
[29] Z.A. Alothman, A review: fundamental aspects of silicate mesoporous materials,
Materials 5 (12) (2012) 2874–2902, https://doi.org/10.3390/ma5122874.
[30] K.S. Kanakikodi, S.R. Churipard, A.B. Halgeri, S.P. Maradur, Catalytic tertiary
butylation of phenol over sulfonated mesoporous polymer catalyst (MP-SO3H):
exceptional selectivity towards 2,4-di-t-Butylphenol, Microporous Mesoporous
Mater. 286 (2019) 133–140, https://doi.org/10.1016/j.micromeso.2019.05.043.
[31] N.J. Venkatesha, Y.S. Bhat, B.S. Jai Prakash, Delaminated BEA zeolite for the
selective synthesis of five membered cyclic acetal from glycerol under ambient
conditions, RSC Adv. 6 (2016) 18824–18833, https://doi.org/10.1039/
C6RA01437B.
[40] G.H. Robinson, S.A. Sangokoya, F. Moise, W.T. Pennington, An unusual trigonal-
bipyramidal coordination of aluminum by an open-chain tetradentate amine.
Synthesis and molecular structure of [Al(CH3)][C8H19N4][Al(CH3)2],
Organometallics 7 (8) (1988) 1887–1889, https://doi.org/10.1021/
om00098a039.
[41] L. Sacconi, Five coordination in 3d metal complexes, Pure Appl. Chem. 17 (1968)
95–127, https://doi.org/10.1351/pac196817010095.
[42] J.A. van Bokhoven, H. Sambe, D.C. Koningsberger, D.E. Ramaker, A NEXAFS study
on the coordination geometry of aluminum in minerals and Y zeolite. Comparison
of experiment and theory, J. Phys. 7 (1997) 835–840, https://doi.org/10.1051/
jp4:1997254.
[43] F.R. Chen, J.G. Davis, J.J. Fripiat, Aluminum coordination and Lewis acidity in
transition alumina, J. Catal. 133 (1992) 263–278, https://doi.org/10.1016/0021-
9517(92)90239-E.
[32] D. Arias, D. Escalante, C.M. Lpez, F.J. Machado, B. Mkndez, D. Moronta, M. Pinto,
V. Sazo, M.M.R. De Agudelo, On the nature of acid sites in substituted
aluminophosphate molecular sieves with the AEL topology, J. Mol. Catal. Chem.
122 (2–3) (1997) 175–186, https://doi.org/10.1016/S1381-1169(97)00028-9.
[33] T.T.P. Cheung, K.W. Willcox, M.P. Mcdaniel, M.M. Johnson, C. Bronnimann,
J. Frye, The structure of coprecipitated aluminophosphate catalyst supports,
J. Catal. 102 (1) (1986) 10–20, https://doi.org/10.1016/0021-9517(86)90135-1.
[34] M. Eden, NMR studies of oxide-based glasses, Annu. Reports Prog. Chem. - Sect. C
(Physical Chemistry) 108 (1) (2012) 177–221, https://doi.org/10.1039/
c2pc90006h.
[44] S. C Kohn, R. Dupree, M.G. Mortuza, C.M.B. Henderson, NMR evidences for five-
and six- coordinated Aluminium fluoride complexes in F bearing Aluminosilicate
glasses, Am. Mineral. 76 (1991) 309–312. http://www.minsocam.org/ammin/am7
6/am76_309.pdf.
[45] L. Zhang, H. Eckert, Sol-gel synthesis of Al2O3-P2O5 glasses: mechanistic studies by
solution and solid state NMR, J. Mater. Chem. 14 (10) (2004) 1605–1615, https://
doi.org/10.1039/B400651H.
[46] S. Wegner, L. Van Wu, An advanced NMR protocol for the structural
characterization of aluminophosphate glasses 32 (2007) 44–52, https://doi.org/
10.1016/j.ssnmr.2007.07.004.
[35] L. Delevoye, L. Montagne, Phase identification and quantification in a devitrified
glass using homo- and heteronuclear solid-state, NMR 42 (2005) 5289–5291,
https://doi.org/10.1039/b511207a.
´
[47] H. Nagashima, C. Martineau-Corcos, G. Tricot, J. Trebosc, F. Pourpoint, J.
[36] F.M. Bautista, J.M. Campelo, A. Garcia, D. Luna, J.M. Marinas, A.A. Romero,
A1PO4-A12O3 catalysts with low alumina content I, Structural and textural
characterization of catalysts obtained with aqueous ammonia 96 (1993) 175–199,
https://doi.org/10.1016/0926-860X(90)80009-4.
P. Amoureux, O. Lafon, Recent developments in NMR studies of
aluminophosphates, Annu. Rep. NMR Spectrosc. 94 (2018) 113–185, https://doi.
org/10.1016/bs.arnmr.2017.12.004.
[48] D.A. Skoog, M. West, F. James Holler, R. Stanley Crouch, Fundamentals of
Analytical Chemistry, eighth ed., Cengage Learning, USA, 2004.
[37] J.G. Longstaffe, B. Chen, Y. Huang, Characterization of the amorphous phases
formed during the synthesis of microporous, material AlPO4-5 98 (2007) 21–28,
https://doi.org/10.1016/j.micromeso.2006.08.009.
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