Physicochemical studies and heterogeneous hydroxylation of benzene on FeII, FeIII-Semicarbazone/zeolite-y clathrates

Article abstract of DOI:10.1080/15533174.2015.1031038

Complexes of Fe(II) and Fe(III) with semicarbazone (SALSC) derived from salicylaldehyde and semicarbazide hydrochloride have been trapped in zeolite-Y super cages using "ship-in-a-bottle technique." The synthesized new clathrates have been structurally ch

Full text of DOI:10.1080/15533174.2015.1031038

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry (2015) 45, 1632–1641
Copyright © Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2015.1031038
Physicochemical Studies and Heterogeneous Hydroxylation
of Benzene on FeII, FeIII-Semicarbazone/Zeolite-Y Clathrates
AYMAN H. AHMED1 and M. S. THABET1,2
1Department of Chemistry, Faculty of Science, Al-Azhar University, Nasr City, Cairo, Egypt
2Chemistry Department, Faculty of Science, Jazan University, Jazan, Saudi Arabia
Received 7 December 2011; accepted 5 February 2013
Complexes of Fe(II) and Fe(III) with semicarbazone (SALSC) derived from salicylaldehyde and semicarbazide hydrochloride have
been trapped in zeolite-Y super cages using “ship-in-a-bottle technique.” The synthesized new clathrates have been structurally
characterized by various physicochemical techniques, in addition to surface area and nitrogen adsorption studies. The Fe(II) and Fe
(III) ions inside zeolite-Y are coordinated with SALSC ligand via the (CDN), phenolic (OH), and deprotonated enolized (CDO)
groups forming mononuclear structures. The results pointed out that the lattice oxygen zeolite was involved in coordination and the
intrazeolitic ferrous and ferric semicarbazone complexes have prevailing octahedral configurations with noticeable high thermal
stability. The catalytic activity of the new hybrid materials was checked for hydroxylation of benzene at 80ꢀC using H2O2 as oxidant
to give phenol.
Keywords: microporous materials, ship-in-a-bottle technique, hydroxylation of benzene
Introduction
bi- or tridentate ligands through their azomethynic nitrogen,
carbonyl oxygen and/or phenolic oxygen atoms.[19,20] Despite
Molecular sieves are adsorbents consisting of crystalline alu- Y-zeolite entrapped Mn(II,III)-salicylaldehyde semicarbazone
minosilicates (zeolites) and clay. The main characteristic of complexes having been recently investigated,[20] the literature
zeolitic molecular sieves is their well-defined pore size and uni- survey disclosed that iron(II,III)-salicylaldehyde semicarba-
form porous structure. Nowadays, zeolite materials in which zone/Y have neither been synthesized nor studied until now.
transition metal complexes have been encapsulated into the Nevertheless, the chemistry and pharmacological applications
aluminosilicate framework have been of considerable interest of semicarbazones and/or their iron(II, III) complexes were
due to its wide applications in catalysis as heterogeneous cata- extensively investigated,[20,21] they have not been tested in
lysts.[1–7] By this way it is possible to perform catalytic reac- catalysis. Although the metal complexes incorporated in zeo-
tions combining the advantages of homogeneous and litic moiety may show better catalytic activity and selectivity
heterogeneous catalytic system.[8] On the other hand, incorpo- than the corresponding free complexes used as homogeneous
ration of metal complexes inside the zeolite enhances the catalyst owing to their dispersion in zeolite,[22] sometimes the
chemical and thermal stability of the occluded complexes reversing may occur due to the constraints in the formation of
owing to the zeolite shielding.[9–13] In many cases, zeolite the intermediate complex.[5]
inclusion is applied to avoid the polymerization and to
Phenol is a versatile chemical used primarily in the forma-
strongly reduce the self-oxidation tendency of the complexes. tion of phenolic resins, as a solvent for petroleum refining
Literature survey in this trend referred to the encapsulation of and in the production of nylon and other synthetic fibers. The
different types of ligands, such as porphyrins, phthalocya- well-known antiseptic properties of phenol promote addition
nines, Schiff bases, (poly) pyridines, amidate, and polyaza industrial and medical applications. Hydroxylation of ben-
macrocyclic models, and in addition we have recently pub- zene is an attractive and defying field. Direct hydroxylation
lished iron(II, III) complexes of hydrazone.[4–7,14–18] Semicar- of benzene to phenol is one of the most difficult problems in
bazones are discriminating hydrazones attracted the attention the field of organic synthesis.[23] Currently, the industrial
of chemists because of their medicinal and analytical impor- route to prepare phenol is the Cumene process, which consists
tance. They are versatile ligands, bearing suitable donor of three steps and produces acetone as a by-product.[24] For
atoms for coordination with metals. In particular, they act as this obvious reason, a one-step process for the production of
phenol is highly preferred. In certain cases further oxidation
occur leading to para-benzoquinone, which is an important
Address correspondence to Ayman H. Ahmed, Department of
intermediate in the manufacture of petrochemicals, agro-
chemicals and plastics.[24] Thus, the main objective of metal
Chemistry, Faculty of Science, Al-Azhar University, Nasr City,
Cairo, Egypt. E-mail: ayman_haf532@yahoo.com
Color versions of one or more figures in this article can be found
pared and capable of oxidizing a harmful benzene to an
online at www.tandfonline.com/lsrt.
catalysis is to search an efficient system, which is easily pre-

FeII, FeIII-Semicarbazone/Zeolite-Y Clathrates
1633
industrial benefit material (phenol) with high selectivity. and DTA) under flowing dry nitrogen were carried out with a
Many researchers attempted a one-step hydroxylation of ben- DTG-60 H thermal analyzer at Cairo University, Egypt. The
zene using N2O, t-BuOOH, and peroxyacids as oxidants; surface texture characteristics obtained from nitrogen adsorp-
ascorbic acid and acetic acid as promoters; and homogeneous tion isotherms were determined at ¡196ꢀC using a conven-
and heterogeneous metal complexes as catalysts.[25] These tional volumetric apparatus. The specific surface areas (SBET
,
oxidants are widely used but are undesirable reagents as their St) were estimated by BET-plots and t-curves where the sam-
by-products, whereas homogeneous catalysts are difficult to ples were thermally degassed at 200ꢀC for 2 h prior to the
recover and costly. Direct liquid phase hydroxylation of ben- adsorption measurements.
zene using H2O2 as an oxidant catalyzed by zeolite catalysts
under the optimum reaction conditions gave only one
product (i.e., phenol).
Preparations
The context of this work aims to prepare new heteroge-
neous catalysts by encapsulating Fe(II) and Fe(III)-semicar-
bazone complexes in zeolite-Y, which are characterized by
different physicochemical techniques. The composition and
structure of the modified zeolites are identified and tested for
hydroxylation of benzene using hydrogen peroxide as oxi-
dant. Also, the effect of zeolite environment on the complexa-
tion process has been studied. Texture of the zeolite encaged
iron-salicylaldehyde semicarbazone (SALSC) complexes is
described and compared with that of the corresponding iron
(II) zeolite showing the possibility of using these new materi-
als in different catalytic applications.
SALSC
The ligand was prepared according to the method described
in Patole et al.[27] Methanolic solution of white semicarbazide
hydrochloride (11.15 g, 0.1 mol) and salicylaldehyde (10 g,
0.1 mol) was refluxed in the presence of sodium acetate cata-
lyst (13.6 g, 0.1 mol) for 3 h under constant stirring. A white
product (desired ligand) separated immediately was collected
by filtration, washed repeatedly by ethanol to remove any
excess of reactants and finally dried in furnace at 80ꢀC for
3 h. Worthy mention, the reactants (semicarbazide hydro-
chloride, mp D 175–177ꢀC, and salicylaldehyde) are freely
soluble in ethanol while the product (salicylaldehyde semicar-
bazone, mp D 222–224ꢀC) is insoluble. The structure of the
resulting ligand shown in Figure 1 was ascertained by IR
spectroscopy and its purity was checked by mass spectrome-
try which exhibited a molecular ion peak (MC value) at
m/z D 179; calcd. D 179.24 (Figure 1). Anal. Calcd. For
SALSC (C8H9N3O2): C, 53.63; H, 5.06; N, 23.45. Found: C,
53.36; H, 5.06; N, 23.17%. 1H NMR (DMSO-d6, 300 MHz):
d (ppm) D 10.2 (s, 1H, NH), 9.8 (s, 1H, OH), 8.2 (s, 1H,
CHDN), 6.4 (s, 2H, NH2) and 6.8–8.1 (m, 4H, Ar-H).
SALSC is air stable, soluble in DMF and insoluble in water
and most of organic solvents (e.g., ethanol, chloroform, ace-
tone, ether, benzene).
Experimental
Materials and Methods
All the chemicals and solvents employed were of AR quality
and used without further purification. The chemicals also
include ferrous sulfate heptahydrate and NaY zeolite (Lot
No. D1-9915, HSZ–320N–NAA), which was obtained from
Toyota Company Ltd., Japan. The parent NaY zeolite has
Si/Al molar ratio of 2.79, which corresponds to a unit cell
formula Na50.5Al50.5Si141O383.240 H2O. Melting point was
determined in open capillary tube using Griffin, George melt-
ing point apparatus and is uncorrected. The elemental analy-
sis for carbon, hydrogen and nitrogen was performed on
Perkin Elmer 2400 at Microanalytical Unit of Cairo Univer-
sity, Egypt. The metal contents (wt%) were determined by
complexometric titration of EDTA using xylenol orange indi-
cator, hexamine buffer and sodium fluoride as a masking
agent for the interfering aluminum ions result from disinte-
gration of the zeolite framework.[26] IR spectra were recorded
as KBr pellet in the range 200–4000 cm¡1 on a Mattson 5000
FT-IR spectrometer using KBr as blank. The 1H NMR spec-
tra were recorded on a Varian Mercury VX-300 NMR spec-
trometer at 25ꢀC using DMSO solvent and TMS as an
internal standard. Mass spectrum of the ligand was recorded
on Shimadzu GC.MS-QP 1000 EX using the direct inlet sys-
tem at Cairo University, Egypt. UV-Vis spectra of the sam-
ples in Nujol mull were measured in the range 200–1100 nm
using a Perkin-Elmer lambda 35 UV-Vis Spectrophotometer.
The mass susceptibility of the solid materials (xg) was mea-
sured with a magnetic susceptibility balance of Johnson Mat-
they Sherwood models. X-ray diffractograms were recorded
Iron-exchanged zeolite (FeII-Y)
FeII-Y was generally prepared via the reversible ion exchange
process simplified by the following equation[14]
:
NaI ¡ YðsÞ C FeIIðaqÞ ! FeII ¡ YðsÞ C 2NaIðaqÞ
using a Philips PW1840 X-ray powder diffractometer with
Fig. 1. Mass spectrum of SALSC accompanied with its chemical
ꢀ
Cu Ka target (λ D 1.54 A). The thermal analyses (TG, DTG, structure.

1634
Ahmed and Thabet
Seven grams of unpromoted NaY were weighed out and dried in air at 100ꢀC for 4 h. In comparison to a beige FeII-Y
placed in 1500 mL of 0.01 M ferrous sulfate solution using and a white SALSC, a dirty and dark green colors of
deoxygenated water and sodium dihydrogen phosphate FeII(SALSC)/Y and FeIII(SALSC)/Y, respectively, have
buffer. The pH of the resulting solution was 4 and the ion been acquired. These color changes are regarded as a satisfac-
exchange process was executed at room temperature for 48 h tory argument for the immobilization of Fe(II, III)-semicar-
with continuous stirring. The powder was suction filtered off, bazone complexes inside the zeolite pores.
washed thoroughly with distilled water to remove the excess
Catalytic Activity: Hydroxylation of Benzene
of adsorbed or unreacted ions till the washing solution was
free from any Fe(II) ions. The FeII-Y zeolite was then dried
The catalytic reaction was carried out using 50 mL benzene
under vacuum at 80ꢀC and finally stored under nitrogen until
in a neck 100 mL round bottom flask connected with a reflux
required for use. The unit cell formula of metal-exchanged
condenser at 80ꢀC and under stirring for 2 h. The solid cata-
zeolites shows 19.9 atoms of iron dispersion per unit cell
lyst (0.15 and 0.5 g/L) was suspended in the solution while a
(Fe19.9Na11.2Al50.5Si141O383.184 H2O). Metal ion exchange at
continuous stirring. After the reaction mixture was heated to
around 78% leads to 6.89 wt% of metal loading in zeolite and
the desired temperature, a hydrogen peroxide (1 mL of 30 v/
the number of water molecules (184) is proposed on the basis
v%) was added to initiate the reaction. Aliquots were with-
of the percent weight loss of all zeolite water (20.54) detected
from TG, DTG thermograms.
drawn during the reaction at selected time, filtered by means
of 0.2 m nylon membrane to analyze the reaction mixture.
Phenol was detected and measured by UV absorbance at a
Clathrates (FeII, FeIII(SALSC)/Y)
wavelength 510 nm using a standard 4-aminoantipyrine col-
orimetric method according to Valkaj et al.[29]
Zeolite encapsulated Fe(II) and Fe(III)-semicarbazone were
prepared using a solid-solid interaction technique by diffusing
Results and Discussion
SALSC hydrazone through the pores of preactivated FeII-Y.
The consequential large and rigid complexes become unable
to escape from the zeolite cage (FL method). For this pur-
pose, an amount of FeII-Y (2.0 g) is activated by heating to
remove the water trapped within their cage-like network. The
heating process was executed in a Pyrex shrink tube to 200ꢀC
for 2 h in a vacuum of 10¡4 Torr and after cooling, FeII-Y
was allowed to use. By this way, SALSC can reach to the
most of FeII ions (FeII-Y) and is competently to interact with
them. For preparing FeII(SALSC)/Y, approximately 1.0 g of
activated FeII-Y was intimately mixed with excess of SALSC
ligand (2.0 g) in a glove bag under nitrogen atmosphere. The
resulting mixture was placed horizontally in a shrink furnace
tube, evacuated again for 10 min at 10¡4 Torr and then fol-
lowed by heating with constant stirring at 140ꢀC for 4 h.
Accommodation of FeIII(SALSC) in NaY is accomplished
by identical method except the intimate mixture [FeII-Y C
SALSC] was allowed to heat at 140ꢀC in open test tube (in
air) instead of using vacuum condition in order to oxidize Fe
(II) ions to Fe(III).[14,16,28] After cooling to room tempera-
ture, the two samples [Fe(II), Fe(III)-semicarbazone com-
plexes] were washed thoroughly in a beaker by DMF and
then hot successive portions of ethanol, acetone and methy-
lene chloride. Soxhlet extracted with ethanol was carried out
for 48 h at normal conditions and the samples were finally
Physical and analytical data of the free SALSC ligand, Fe
(II)-exchanged zeolite, and zeolite encapsulated iron(II,III)
complexes are listed in Table 1. The prepared composites are
insoluble in common organic solvents. The slightly difference
between calculated and found metal to carbon (M/C) ratio
in case of FeIII(SALSC)/Y (Table 1) is plausibly due to insig-
nificant unreacted part of Fe(II) ions in zeolite voids.
Stereochemical Structure of Zeolite Encapsulated Iron-
Semicarbazone Complexes
The significant IR bands of the free SALSC along with its
corresponding FeII(SALSC)/Y and FeIII(SALSC)/Y as well
as the parent NaY were summarized and assigned in Table 2.
FeII(SALSC)/Y and FeIII(SALSC)/Y are explicitly domi-
nated by Y bands assignable to surface hydroxyl group, inter-
nal and external vibrations of the framework TO4 tetrahedral
(T D Si or Al). FeII-Y spectrum did not show any significant
shift in their external linkage vibrations indicating the
absence of any interaction between Fe(II) ion and the frame-
work structure where FeII-Y is found in zeolite as [Fe
(H2O)6]2C [14]
On the other hand, It was noticed from the
.
data depicted in Table 2 and shown in Figure 2 that there is
a somewhat distortion or dealumination in the zeolite moiety
Table 1. Physical and analytical data for SALSC and the prepared zeolites
Sample
label
Fe
wt%)
M.p/Dec. temp.
(ꢀC)
M/C Found
(Calcd.)
Product
assignment
Color
xgc.g.s
SALSC
White
Beige
Dirty green
Dark green
—
6.89
4
222¡224a
—
—
—
—
FeII-Y
>300
1.06 £ 10¡5
1.05£10¡5
0.95£ 10¡5
[Fe(H2O)6]2C
FeII(SALSC)/Y
FeIII(SALSC)/Y
>300
>300
0.13(0.13)
0.15(0.13)
[Fe(SALSC)(H2O/Oz)3]-Y
4.9
[Fe(SALSC)(H2O/Oz)3]C-Y
Note: (M.p) melting point; (Dec. temp) decomposition temperature; (xg) mass magnetic susceptibility.
amelting without decomposition.

1635

1636
Ahmed and Thabet
for participating the oxygen of zeolite lattice in coordination,
that is, Fe-O(zeolite) linkage was formed in both FeII
(SALSC)/Y and FeIII(SALSC)/Y samples. In addition,
SALSC acts as a binegative tridentate ligand (O, N, Oꢁꢁ)
forming a five-membered chelate ring around the central iron
atom as reported in some papers.[19,31] This behavior is sug-
gested on the following bases: (a) The disappearance of the
n,d(NH) of (ꢁꢁCOꢁꢁNHꢁꢁ) in the free ligand upon complex-
ation indicates the deprotonation of ligand and refers to its
conversion to (ꢁꢁC(OH)DNꢁꢁ) prior to coordination. (b)
The higher frequency shift of the band assigned to the pheno-
lic n(CꢁꢁO) stretching by chelation indicates the coordination
through the phenolic oxygen. (c) The lower shift in the azo-
methine (CDN) and the carbonyl (CDO) groups upon com-
plexation substantiates the involvement of the azomethine in
coordination and a considerable change in the bond order of
the CDO group.[32]. Undoubtedly, this lower shift besides the
appearance of MꢁꢁO or MꢁꢁN bands manifest the interac-
tion between the ligand and the exchanged metal ion. Fur-
thermore, a medium band at 1030 cm¡1 due to n(C-N) or
n(N-C-N)[33] in the ligand is suggested to be shifted to higher
frequency (1050–1150 cm¡1) in the spectra of the SALSC
samples. The possibility of a-nitrogen coordination is ruled
out because of the considerable strain and incapability to
form a stable five or six membered chelate ring around the
metal ion in such case. Coordinated water in FeII(SALSC)/Y
and FeIII(SALSC)/Y was supposed based on the appearance
of the bands at 1413 and 1402 cm¡1, respectively, assigned to
d(OH) in plane.[34] Other bands ascribed to coordinated water
that would be expected to appear at 600–1100 and 1600–
1680 cm¡1 were overlapped by Y bands.
The electronic spectral data for SALSC, FeII(SALSC)/Y
and FeIII(SALSC)/Y have been studied and shown in Fig-
ure 3. The electronic spectrum of the zeolite encapsulated Fe
(II) complex, FeII(SALSC)/Y, exhibited a M!L charge
transfer at 606 nm suggesting an octahedral environment
Fig. 2. FT-IR spectra of (a) NaY, (b) FeII-Y, (c) FeII(SALSC)/
Y, (d) FeIII(SALSC)/Y, and (e) SALSC.
assigned to SALSC samples owing to the variation in the around the Fe(II) ions.[35] This band is due to the excitation of
positions and features of some structure-sensitive bands. This a metal d (t2g) electron to the empty p* orbital on the ligand.
can be explained as follows: the weak pore opening band cen- For FeIII(SALSC)/Y, the electronic spectrum showed a board
6
4
tered at 390 cm¡1 in NaY varied its shape and position band around 580 nm assigned to A1g! T2g transition,
appearing as medium (with a blue shift) at 376 (FeII characteristic to an octahedral geometry.[14,21c] Both FeII(-
(SALSC)/Y) and 384 (FeIII(SALSC)/Y) cm¡1. Besides, it SALSC)/Y and FeIII(SALSC)/Y exhibited two bands near
was noticed in the case of FeIII(SALSC)/Y that (a) the most 365 and 343 nm mainly attributed to (Al O) charge transfers
interesting bands assigned to nasym,
TꢁꢁO revealed a in Y zeolite.[36] The observation of the high intensity band at λ
sym
noticeable change in their features and positions indicating a D 343 nm related to the framework Al O units, which are
strong interaction between the metal ion and the zeolite highly polar and easily removed by dealumination or dehy-
framework, (b) the shoulder nasymTꢁꢁO band located at droxylation, besides its appearance at higher wavelength com-
1140 cm¡1 in NaY appeared at 1145 cm¡1 as strong, (c) the pared with NaY (335 nm) and FeIIY (325 nm)[11,14] excludes
nsym TꢁꢁO band centered at 787 cm¡1 appeared at 795 cm¡1 the dealumination and substantiates the distortion. This result
by shift of 8 cm¡1. Based on the fact that, variation in the fea- coincides with the obtained IR and XRD data.
tures and positions of the structure sensitive bands appears to
In practice, calculations of the effective magnetic moments
be related to framework structure characteristics,[30] it is sug- (meff) require the knowledge of the molecular weight of the
gested that a sort of distortion or dealumination has been zeolite samples. As the complexes formed in the zeolite are in
occurred in FeII(SALSC)/Y and FeIII(SALSC)/Y. Owing to bonding with the lattice oxygen of zeolite as substantiated by
these samples are still crystalline as obtained from IR and IR data, the molecular weights for these complexes are unde-
confirmed by the XRD later, the most probable to occur is termined and subsequently it becomes impracticable to calcu-
the distortion. Perhaps the lower shift in the position of the late the meff. Nonetheless, magnetic measurement is benefit
structure sensitive bands indicates a weak deformation in the because it gives some information on the sample nature. For
framework. This deformation may be taken as an evidence FeII(SALSC)/Y, as FeII has a d6 electronic configuration,

FeII, FeIII-Semicarbazone/Zeolite-Y Clathrates
1637
unpaired electrons, are believed to be correct near 4.9 and
5.9 B.M., respectively (spin only values).
In the light of the forgoing results discussed previously, the
ship-in-a-bottle representation of the immobilized Fe(II,III)-
SALSC chelates inside the zeolite matrix are shown in
Scheme 1. Despite the chemical structures are well authenti-
cated by single crystal X-ray, we cannot isolate a single crys-
tal from heterogonous materials. If we try to separate the
encapsulated metal-SALSC complexes or prepare them in
free state, we expect to obtain differ complexes because (a)
the resulting data ascertained the involvement of zeolite oxy-
gen in coordination, (b) the encapsulated Metallo-SALSC
are cationic complexes compensated by negative charges on
zeolite framework, and (c) In many cases, the entrapment of
the complex in the zeolite leads to construction of new com-
position and structure differs from the free one owing to the
zeolite restriction. According to the chemical data, the load-
ing of the zeolites with SALSC complexes can be nearly pro-
Fig. 3. Electronic spectra of SALSC, FeII(SALSC)/Y and FeIII
(SALSC)/Y.
strong field ligand causes spin pairing. Spin pairing also results posed. As both FeII- and FeIII–SALSC structures imply one
in diamagnetic complexes. The paramagnetic nature of this metal ion (Scheme1), number of complex molecules for each
octahedral complex indicated from the xg value (Table 1) sug- kind will be 19.9 per unit cell of the Y-type zeolite. Subse-
gests a high spin arrangement with four unpaired electrons quently, the number of complexes per one super cage is
[(t2g)4 (eg)2]. Furthermore, it ascertains that SALSC ligand approximately to 2.5. This fact is assumed on the following
has a weak field, and thus the FeII(SALSC)/Y is expected to bases: (a) The remarkable legation properties of aryl semicar-
has a lower crystal field stabilization energy and subsequently bazones by which the SALSC ligand might extract major of
less stability. In case of FeIII(SALSC)/Y, FeIII has a d5 config- the exchanged iron ions from the hexagonal prism or b-cage
uration with weak field SALSC ligand (as evident in FeII(- toward the super cages. (b) There are eight super cages per
SALSC)/Y) and thus the complex will have a high spin unit cell of zeolite Y.
arrangement with five unpaired electrons [(t2g)3 (eg)2]. Also
the stability is not higher as with strong field ligand such as
Powder X-Ray Diffraction Studies
CN¡ or SCN¡. This expectation is in consistent and not con-
flicting with its paramagnetic xg value (Table 1). The expected
room temperature magnetic moments for FeII(SALSC)/Y
and FeIII(SALSC)/Y samples, corresponding to four and five
XRD technique can provide a conclusive argument for
the immobilization of FeII, FeIII(SALSC) complexes inside
the zeolite cages. X-ray powder diffraction patterns of FeII-Y
Sch. 1. Ship in a bottle representation of octahedral iron-semicarbazone complexes in zeolite-Y.

1638
Ahmed and Thabet
of the investigated hybrid materials [FeII(SALSC)/Y and
FeIII(SALSC)/Y] have been studied and shown in Figure 5.
TG curve of FeII(SALSC)/Y showed three stages
of decomposition within the region 40–190, 190–320, and
320–530ꢀC. DTA curve showed also three weight loss endo-
thermic peaks at midpoints 87, 327, and 510ꢀC. These three
peaks noticed in both TG and DTA curves are returned to
the water removal from the zeolite voids. TG curve of FeIII
(SALSC)/Y exhibited also three decomposition stages start-
ing at 40, 190, and 330ꢀC for the desorption of water from
the interacrystalline voids of zeolite-Y. DTA curve did not
show any exothermic peak assignable to the decomposition
of the immobilized ferric complex. The obscure of any exo-
thermic weight loss peak in the DTA curves of FeII(SALSC)/
Y and FeIII(SALSC)/Y within the range 30–800ꢀC indicates
the thermal stability of the discriminated iron-semicarbazone
complexes up to 800ꢀC. Despite the neat FeII, FeIII-SALSC
complexes were not thermally studied before, CuII, CoII-2-
hydroxynaphthaldehyde-semicarbazone complexes, which
seem to be almost synonymous with the investigated host/
guest materials in their thermal behavior are stable up to 165
and 141ꢀC, respectively.[21] This conspicuously indicates that
the immobilization of the complexes inside the zeolite cages
enhances their thermal stability to a noticeable degree due to
the zeolite shielding effect. In fact, execution the percentage
Fig. 4. X-ray powder diffraction patterns of (a) FeII-Y, (b) FeII
(SALSC)/Y, and (c) FeIII(SALSC)/Y.
along with the zeolite encapsulated iron complexes are shown decomposition of components of Fe(II) and Fe(III)-SALSC
in Figure 4 and explained in accordance to the published complexes and correlate it with total molecular weight of
NaY.[14] The data are unambiguously showed that the crystal- complexes would expect to be inaccurate owing to the molec-
linity of zeolite Y is almost preserved after entrapment of the ular weights of these samples are undetermined. We deal with
metal ion or SALSC complexes in it. Diffraction characteristic composite materials and we may have several numbers of
feature for any new phases in the diffractograms up to two encapsulated complex molecules dispersed in one unite cell.
theta equal 70 was not detected indicating the absence of any Moreover, the number of the complexes in the unite cell or
new interfering substances on the external surface of NaY. As super cages cannot be detected exactly. M/C atomic ratios of
the XRD is a surface analytical technique and there is a differ- MIII(SALSC)/Y samples (Table 1) refer to the existence of
ence in colors of SALSC, NaY and FeII-Y samples by occlu- unreacted part of Fe(III) ions in zeolite voids, in addition the
sion, the obscure of new peaks due to neat complex provides ligand may be unable to react with all of the exchanged metal
conspicuous evidence to residing of the metal complexes inside ions specially the metal ions may occupy different sites in
the zeolite cavity and not on the zeolite surface. Furthers, the NaY. Although, the encapsulated complexes did not decom-
relative intensities of the 311, 220 reflections in published NaY pose within the utilized region of thermal measurement, the
(I220 > I311) were reversed upon introducing the Fe(II) ions or thermal stability order for the investigated iron complexes is
their related complexes (i.e., I311 > I220). This can be inter- expected to be FeIII(SALSC) > FeII(SALSC). The reason for
preted on the basis the replacement of NaC in NaY by large this is fairly due to the greater magnitude of charge on the fer-
cations (Fe2C) or complex molecules [FeII(SALSC) and FeIII(- ric ions, which leads to the greater of the electrostatic attrac-
SALSC)] leads to disturb the random distribution of small tion with ligand and zeolite, and thus the more stable the
extra framework cations[17] and this change in the location of resulting complex is. Furthers, Fe3C has a half field d orbital
small cations affects the relative intensities of 311, 220 peaks. (d5 configuration) which gives a high stability for ferric com-
This of course indicates that the entrapped large iron com- pounds in contrast to the Fe2C ion which has a d6
plexes have displaced the sodium ions from their random posi- configuration.
tions in the super cages to locations of b-cage and center of a
single six-ring (S6R)[37] residing inside the super cage of zeolite
Surface Texture Characteristics
Y. It is worth to mention that, the changes in the charge distri-
bution within the cavities and channels alter the adsorption The texture of a catalyst surface, namely, the specific surface
behavior as well as the catalytic activity.
area and pore geometry are of prime importance in determin-
ing the activity, sometimes, also the selectivity of the solid
catalyst.[38] From this view, surface texture of the synthesized
samples should be determined. Indeed, nitrogen adsorption
Thermogravimetric Studies
Thermal analysis can provide some information about the studies of NaY and FeII-Y were previously reported and the
influences of zeolite shielding on the thermal stability of met- results showed that the N2-isotherm is a typical of micropo-
allo-SALSC complexes. Thermograms (TG, DTG and DTA) rous solid for NaY[11] and a medium type between I and II of

FeII, FeIII-Semicarbazone/Zeolite-Y Clathrates
1639
Fig. 5. TG, DTG and DTA curves of the zeolite encapsulated iron complexes.
Brunauer classification[14] with a hysteresis loop for FeII-Y.[3] still retained, definitely, showing that they are still porous
The presence of this loop indicates the porous nature of FeII- materials. In comparison to the published NaY and FeII-Y
Y solid. Vl-t plots of NaY and its Fe-exchanged zeolite isotherms,[11,14] FeII(SALSC)/Y and FeIII(SALSC)/Y exhib-
revealed that the parent NaY is characterized by the domina- ited a decrease in the N2 adsorption capacity pointing to the
tion of narrow pores while its metal exchanged acquires wide filling of the zeolite pores by iron complexes. Comparing of
porosity. The reason of this is due to the displacement of FeII(SALSC)/Y sample with FeIII(SALSC)/Y revealed that
2NaC ions from its original sites by one FeII ion. This revision the latter exhibits a decrease in the surface area (SBET) assum-
decreases the number of cations in zeolite nearly to half caus- ing a larger filling of the zeolite pores by Fe(III) complex. On
ing a creation of new type of pores and thus the mesoporsity the other hand, the Vl-t construction of FeII(SALSC)/Y and
is generated as noticed in Table 3. Surface texture of FeII(- FeIII(SALSC)/Y (Figure 6) exhibited a downward deviation
SALSC)/Y and FeIII(SALSC)/Y was studied by nitrogen indicating the domination of narrow pores. As the zeolite
adsorption conducted at ¡196ꢀC and showed a medium type framework structure did not collapse by encapsulation as
between I and II of Brunauer classification.[39] Some surface substantiated from the IR and XRD patterns, the reduction
characteristics of modified zeolites have been depicted in of surface area and total pore volume of Y-zeolite upon intro-
Table 3. The obtained FeII(SALSC)/Y and FeIII(SALSC)/Y ducing the metallo-SALSC complexes provides direct evi-
isotherms did not show hysteresis loops and seem to be dence for the existence of complexes in the nanocages and in
almost synonymous. Perhaps, this foundation indicates the the same time refers to their filling degree. Based on micropo-
none porous nature due to the filling of the zeolite pores by rous volume (Vmic) of FeII-Y, the decrease in the analogous
SALSC complexes, but their large surface areas (Table 3) are of FeII(SALSC)/Y (by 9%) and FeIII(SALSC)/Y (by 31%)
Table 3. Some surface characteristic data of FeII-Y and its corresponding clathrates
Sample
label
BET
St
Vp
Vmic
Vmes
Smic
Swide
Sext
ꢀ
(m2/g)
(m2/g)
(cm3/g)
í(A)
(cm3.g)
(cm2/g)
(m2/g)
(m2/g)
(m2/g)
FeII-Y
743
693
539
746
672
580
0.78
0.73
0.68
26.2
26.3
31.5
0.67
0.61
0.46
0.11
0.12
0.22
638
580
484
105
114
175
139
54
145
FeII(SALSC)/Y
FeIII(SALSC)/Y
Note: (SBET) BET-surface area; (St) surface area derived from V1_t plots; (Smic) surface area of micropores; (Smes) surface area of mesopores; (Sext) external
surface area; (Vp) total pore volume; (Vmic) pore volume of micropores; (Vmes) pore volume of mesopores; (í) mean pore radius.

1640
Ahmed and Thabet
Fig. 7. Phenol concentration as a function of time on 0.15 g/L
sample catalyst.
encapsulated SALSC catalysts compared with FeII-Y is most
probably results from constrains in the formation of the inter-
mediate complex owing to the zeolite restriction and/or
change of the redox potential of the metal in the SALSC com-
plex. Also, it can be concluded that the pore blockage of
Y-zeolite by immobilized SALSC complex (as evidenced
from the larger decrease in surface area; Table 3) leads to a
drop in the selectivity to phenol. Interestingly, the hydroxyl-
ation activity of benzene was explicitly enhanced by increas-
ing the dosage of catalysts from 0.15 g/L to 0.5 g/L
(Figures 7 and 8). This is related to the increment of active
sites which cause an enhancement for the hydroxylation reac-
tion. The mechanism for the catalytic reaction is suggested to
Fig. 6. Vl-t plots of the zeolite encapsulated iron-SALSC
complexes.
must be attributed to the large volume of the encapsulated be similar to that reported in Qi et al.[40] and can be repre-
complexes. The generated low value of Sext in case of FeII(- sented by Scheme 2. The hydroxylation reaction of benzene
SALSC)/Y compared with FeII-Y and FeIII(SALSC)/Y may over FeII(SALSC)/Y and FeIII(SALSC)/Y species proceeds
be due to the existence of small unreacted part of SALSC or similar to Fenton’s chemistry. This assumption is supported
its FeII-complexes on the external surface. The specific sur- by the observation that: the evolution of O2 from the reaction
face area (St) for these composite materials were determined system suggests the presence of hydroxyl radicals (¢OH) and
and are vary close to (SBET) indicating the correct choice of per hydroxyl radicals (¢OOH) where both of which were gen-
the standard t-curves.
erated upon the interaction of the framework Fe(II) species
with H2O2. The Fe(II) ions in nanoparticles (inactive in
hydroxylation) are probably irreversibly reoxidized to Fe
(III), which are known to be responsible for the total oxida-
tion of benzene.[41] The hesitated variation in the % conver-
Catalytic Activity
Transition metals can act as catalysts because they have sev- sion of phenol with time increased with the reaction time,
eral oxidation states and their ions can bond to a wide range and then there is decrease and further increase can be
of atoms and molecules (e.g., those with lone pair or
p–electrons forming complexes with different numbers of
bonds). This feature gives them a high probability to form
the suitable intermediates required for preceding the catalytic
reactions. The catalytic study of the prepared species was per-
formed at abovementioned conditions as a function of time
and the catalytic results of liquid-phase hydroxylation
of benzene over modified zeolite catalysts are represented in
Figures 7 and 8. The results revealed that FeIII(SALSC)/Y
catalyst showed a slightly increase in its catalytic activity
compared with FeII(SALSC)/Y sample but this behavior is
reversed by time. On the other hand, the parent FeII-Y
showed a high concentration of phenol compared with FeII(-
SALSC)/Y and FeIII(SALSC)/Y. The highest selectivity of
FeII-Y arises from the further oxidation of benzene by Fe(II)
ions. The reduction in the catalytic activity of the zeolite
Fig. 8. Phenol concentration as a function of time on 0.5 g/L
sample catalyst.

FeII, FeIII-Semicarbazone/Zeolite-Y Clathrates
1641
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In the light of recorded experimental data and from converg-
ing arguments made previously we conclude that zeolite-Y
can accommodate iron(II) and iron(III) complexes of SALSC
in the supper cages by FL method without any strain. Spectro-
scopic techniques, elemental analysis, magnetism, thermogra-
vimetric, and XRD patterns as well as surface area and
nitrogen adsorption measurements can identify and character-
ize the molecular structure of the iron species formed in zeo-
lite-Y. The results showed that, the SALSC coordinates to Fe
(II) and Fe(III) ions in a binegative tridentate (O, N, Oꢁꢁ)
manner through the (CDN) and deprotonated enolic
(CꢁꢁOH) and phenolic (OH) groups forming 1M:1L occluded
complexes. The zeolite oxygen (Oz) was involved in coordina-
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