Insights into the active species of Nanoparticle-functionalized hierarchical zeolites in alkylation reactions

Article abstract of DOI:10.1002/cctc.201402555

Supported iron oxide nanoparticles have been incorporated onto hierarchical zeolites by microwave-assisted impregnation and mechanochemical grinding. Nanoparticle-functionalised porous zeolites were characterised by a number of analytical techniques such as XRD, N₂ physisorption, TEM, and surface acidity measurements. The catalytic activities of the synthesised nanomaterials were investigated in an alkylation reaction. The results pointed to different species with varying acidity and accessibility in the materials, which provided essentially different catalytic activities in the alkylation of toluene with benzyl chloride under microwave irradiation, selected as the test reaction. Iron out the creases: Supported iron oxide nanoparticles are incorporated on hierarchical zeolites by microwave-assisted impregnation and mechanochemical grinding. The catalytic activities of the synthesized nanomaterials are investigated in the alkylation of toluene with benzyl chloride under microwave irradiation.

Full text of DOI:10.1002/cctc.201402555

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DOI: 10.1002/cctc.201402555
Insights into the Active Species of Nanoparticle-
Functionalized Hierarchical Zeolites in Alkylation
Reactions
Aida Grau-Atienza,[a] Rafael Campos,[b] Elena Serrano,[a] Manuel Ojeda,[b]
Antonio A. Romero,[b] Javier Garcia-Martinez,*[a] and Rafael Luque*[b]
Supported iron oxide nanoparticles have been incorporated
onto hierarchical zeolites by microwave-assisted impregnation
and mechanochemical grinding. Nanoparticle-functionalised
porous zeolites were characterised by a number of analytical
techniques such as XRD, N2 physisorption, TEM, and surface
acidity measurements. The catalytic activities of the synthes-
ised nanomaterials were investigated in an alkylation reaction.
The results pointed to different species with varying acidity
and accessibility in the materials, which provided essentially
different catalytic activities in the alkylation of toluene with
benzyl chloride under microwave irradiation, selected as the
test reaction.
Introduction
Zeolites are microporous materials that have been investigated
extensively over the past decades because of their important
industrially related applications and implications in heteroge-
neously catalysed processes. Although they are used widely in
many chemical processes, zeolites have been always limited by
the narrow pore structure defined by their microporous crys-
talline framework.[1–3] Many techniques have been used to in-
troduce intracrystalline mesoporosity into zeolites to overcome
the severe diffusion limitations in such materials for the con-
version of bulky molecules.[4–16] Among these treatments, desi-
lication is one of the simplest and cheapest ways to introduce
mesoporosity in zeolites.[4–9] The treatment of zeolites (e.g.
ZSM-5) with moderate Si/Al ratios (typically 25–50)[5] with alka-
line solutions allows the development of intracrystalline meso-
porosity, which improves catalytic performance in hierarchical
zeolites, especially in diffusion-limited processes.[2,3]
ed as highly active catalysts in a range of processes such as al-
kylations, acylations, related acid-catalysed processes and more
recently biomass-derived transformations.[19] However, there
are only few a studies that relate to the functionalisation of hi-
erarchical zeolitic materials and insightful investigations on the
nature of the active sites in heterogeneously catalysed
processes.[19,20]
Nanoparticles are active and stable catalysts once loaded
onto an adequate support because of their small size, which
confers them with a high surface area and a large amount of
exposed active sites, and thereby good contact with reactants.
Nanostructured heterogeneous catalysts are known to over-
come the issues related to both homogeneous and heteroge-
neous catalytic systems, for example, they can be separated
easily from reaction mixtures and show enhanced stability in
comparison with traditional heterogeneous catalysts.[21]
In this work, we have studied the effect of desilication (by
alkali treatment) and subsequent acid treatment in the func-
tionalisation of hierarchical ZSM-5 zeolites with different Si/Al
ratios (15 and 40) with iron oxide nanoparticles. The surface
acid properties of the materials have been investigated and
correlated to the catalytic activity of the materials in the micro-
wave-assisted alkylation of toluene with benzyl chloride. This
process has been reported previously to be preferentially pro-
moted by Lewis-acid sites,[22] which is particularly useful if hier-
archical zeolites are employed[19b] as well as supported iron
oxide nanomaterials.[23a]
An important step in the development of mesoporosity by
desilication is a subsequent acid wash to remove the alumina
debris formed during the first step in which silica is mostly ex-
tracted.[17] Without this acid wash, the zeolite porosity is mainly
blocked by the presence of amorphous alumina-rich debris
species.[18] Hierarchical zeolite-type materials have been report-
[a] A. Grau-Atienza, Dr. E. Serrano, Dr. J. Garcia-Martinez
Molecular Nanotechnology Lab.
Departamento de Quꢀmica Inorgꢁnica
Universidad de Alicante
Crtra. San Vicente s/n, E03690, Alicante (Spain)
Homepage: www.nanomol.es
E-mail: j.garcia@ua.es
Results and Discussion
[b] R. Campos, M. Ojeda, Dr. A. A. Romero, Dr. R. Luque
Departamento de Quimica Orgꢁnica
Universidad de Cꢂrdoba
Hierarchical zeolite catalysts functionalised with iron oxide
have been prepared by desilication and acid treatment of two
commercial zeolites with the further incorporation of iron
oxide nanoparticles under microwave-assisted irradiation and
ball milling.
Edificio Marie Curie (C-3)
Ctra Nnal IV-A, Km 396, 14014, Cꢂrdoba (Spain)
Homepage: uco.es/~q62alsor
E-mail: q62alsor@uco.es
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Figure 2. a) Representative N2 adsorption/desorption isotherms at 77 K and
b) the corresponding pore size distributions of samples with Si/Al=15
(Z15c). Z15c-NaOH was treated with 0.8m NaOH.
Figure 1. a) Representative N2 adsorption/desorption isotherms at 77 K and
b) the corresponding pore size distributions of samples with Si/Al=40
(Z40c). Z40c-NaOH was treated with 0.4m NaOH.
0.8m NaOH, respectively. Accordingly, the BET area decreases
upon reduction of the micropore volume (from 360 for Z15c
to 290 and 85 m2 gÀ1 after treatment with 0.4 and 0.8m NaOH,
respectively), in addition to the increase of the mesopore
volume with the alkaline treatment. The same trend has been
observed for the Z40c zeolite, which indicates that the micro-
pore reduction is related to the presence of amorphous Al-rich
The mesoporous nature of the samples was investigated by
N2 adsorption/desorption at 77 K and TEM. Selected N2 adsorp-
tion/desorption isotherms and the corresponding pore size dis-
tribution of the Z40c- and Z15c-based systems are shown in
Figures 1 and 2, respectively. Based on the isotherms, the tex-
tural parameters were calculated and are listed in Table 1.
Parent zeolites (Z40c and
Z15c) exhibit a type I isotherm
with a narrow knee at low rela-
Table 1. Chemical composition and textural parameters of the synthesised hierarchical catalysts compared to
tive pressures in close agree-
ment with its microporous struc-
ture. The desilication treatment
with NaOH induces a significant
deterioration of the microporous
structure, which is shown by the
amount of N2 adsorbed that de-
creases drastically at low relative
pressures for samples with
a higher NaOH concentration,
that is, 0.8m (Figures 1 and 2
and Table 1). As reported previ-
ously, the surface area and its re-
lationship with the micropore
and mesopore volumes is
a good indication of the quality
of the hierarchical systems.[15,16,18]
For example, the micropore
volume of the parent Z15c zeo-
lite was reduced from 0.14 to
0.11 and 0.01 cm3 gÀ1 after its al-
kaline treatment with 0.4 and
the parent zeolites before and after functionalisation with iron oxide nanoparticles.
[d]
[e]
[e]
[f]
[g]
Sample
Si/Al[a]
Fe[a]
[wt%]
SBET
Smeso
Vmicro
Vmeso
Vporo
[m2 gÀ1
]
[m2 gÀ1
]
[cm2 gÀ1
]
[cm2 gÀ1
]
[cm2 gÀ1
]
Z40c
Z40c-0.4NaOH
Z40c-H
Z40c-0.8NaOH
Z40c-0.8NaOH-HCl
Z15c
Z15c-0.4NaOH
Z15c-0.4NaOH-HCl
Z15c-0.8NaOH
Z15c-H
48.2 (40[b]
–
50.0
–
)
)
–
–
–
–
–
–
–
–
410
380
610
110
430
360
290
500
85
690
520
425
–
210
330
440
110
425
190
150
370
80
590
445
365
–
0.14
0.09
0.16
0.01
0.06
0.14
0.11
0.15
0.01
0.16
0.09
0.07
–
0.16
0.83
1.13
0.70
0.74
0.13
0.28
0.44
0.38
0.89
0.91
0.64
–
0.30
0.92
1.29
0.71
0.80
0.27
0.39
0.59
0.39
1.05
1.00
0.71
–
–
19.5 (15[b]
–
–
–
20.0
38.1
42.3
44.6
22.4
–
–
Fe 0.5/Z40c-H (MW)
Fe 1/Z40c-H (BM)
Fe 1/Z40-H (BM) 7th use
Fe 0.5/Z15c-H (BM)
0.3/0.6[c]
0.7/1.8[c]
0.8
0.3/0.7[c]
355
330
0.06
0.68
0.74
[a] Surface Si/Al mole ratio and Fe contents as determined by XPS after the samples were etched. [b] Theoreti-
cal value according to the manufacturer’s specifications for the uncalcined samples. [c] Fe contents calculated
by ICP-AES analysis of the filtrate after treatment of the samples with phosphoric acid and hydrochloric acid.
[d] BET surface area was estimated by multipoint BET method using the adsorption data in the P/P0 range of
0.05–0.30. [e] “External” surface area and micropore volume were obtained from the isotherms using the t-plot
method. [f] Mesopore volume was obtained from the isotherms using the DFT method. [g] Pore volume was
obtained from the isotherms at P/P0 =0.95.
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fragments and points out the need of the acid washing in the
materials.[15,16,18] Desilicated samples, mainly Z40c-related sys-
tems, also showed enhanced N2 adsorption at relatively high
partial pressures (P/P0 >0.8), which is related to the develop-
ment of interparticle meso-/macroporosity (also deduced from
the pore size distributions shown in Figure 1).
After acid washing, all desilicated samples show N2 adsorp-
tion at intermediate to high relative pressures related to the
development of mesoporosity (Figures 1b and 2b), which is
further confirmed by the increase in the mesopore volume of
the desilicated samples compared with the parent zeolites
(Figures 1 and 2 and Table 1). All samples show type IV iso-
therms with mesopore volumes between 0.44 and 1.13 cm3 gÀ1
and micropore volumes similar to those of the parent zeolites,
typical of hierarchical zeolites (Figures 1 and 2 and Table 1).
Furthermore, an increase in both the BET surface area and the
“external” surface area can be observed clearly in all the hier-
archical zeolites, Z15c- and Z40c-based systems, which is usual-
ly ascribed to the increased mesopore volume. For example,
for the Z15-derived samples, the starting Z15c has a BET sur-
face area of 360 m2 gÀ1, of which 190 m2 gÀ1 is the contribution
of the “external” surface area from t-plot analysis, that is, surfa-
ces areas from the true crystal external surfaces and from inter-
nal voids larger than micropores.[15,16] The hierarchical Z15c-H
zeolite exhibited a BET surface area of 690 m2 gÀ1, which is ap-
proximately 330 m2 gÀ1 higher than that of the parent sample.
A contribution of approximately 590 m2 gÀ1 is derived from the
“external” surface of the hierarchical zeolite, which is approxi-
mately four times higher than that of parent Z15c, in agree-
ment with the increase of mesopore volume (0.13 vs.
0.89 cm3 gÀ1).
Figure 3. TEM micrographs of: (top left) Z40c, (top right) Z15c, (middle left)
Z40c-0.4NaOH, (middle right) Z15c-0.8NaOH, (bottom left) Z40c-H and
(bottom right) Z15c-H. Scale bar=20 nm. Scale bar in the inset=10 nm,
except for Z40c, the scale bar of which corresponds to 20 nm.
TEM images also showed structural features of hierarchical
porous zeolites after NaOH treatment (desilication and open-
ing of the highly crystalline zeolite particles) followed by HCl
treatment (Figure 3).
Two different sets of functionalised materials were synthes-
ised by microwave-assisted (MW) and dry milling (BM) incorpo-
ration of iron oxide nanoparticles. The synthesised materials
(Table 1 and Figures 1 and 2) possessed similar textural proper-
ties, in terms of surface area and pore volume, to the parent
hierarchical zeolites.
Fe materials as measured by inductively coupled plasma
atomic emission spectroscopy (ICP-AES); Table 1). The surface
Fe content (measured by X-ray photoelectron spectroscopy
(XPS) with and without etching) was determined to be approx-
imately a third of the total Fe content in the materials
(Table 1). Surface Si/Al molar ratios (also measured by XPS) re-
mained almost unchanged upon Fe incorporation. The Fe dis-
tribution was also rather homogeneous on the hierarchical
zeolites (even in materials reused several times in the alkyla-
tion reaction) as demonstrated clearly in the SEM mapping
and TEM images (Figures 4 and 5, respectively), although the
presence of iron oxide nanoparticles could not be distinguish-
ed clearly by TEM (Figure 5).
Upon the incorporation of iron oxide nanoparticles, a partial
blockage of the micropores (0.16 vs. 0.06–0.09 cm3 gÀ1) was ob-
served in the hierarchical zeolites, which can be mainly attrib-
uted to iron migration into the micropores (to cause blockage)
as well as the partial collapse of the porous structure,[23b,25] as
observed in the pore size distributions shown in Figures 1 and
2. In any case, a preferential deposition of iron oxide nanopar-
ticles takes place on the external surface of the materials (for
both MW and BM methodologies) as later confirmed in the cat-
alytic activity results. The adsorption at P/P0 >0.6, which is
characteristic of textural interparticle meso-/macroporosity, re-
mains almost identical to those of Z15c-H and Z40c-H for
0.5 wt% Fe catalysts (Figures 1 and 2 and Table 1).
Representative XPS spectra of the Fe-containing materials in
the region that corresponds to the Fe2p peak are shown in
Figure 6. As shown in the XPS spectra of Fe 1/Z40c-H (BM), the
binding energies (BEs) of the Fe2p3/2 and Fe2p1/2 peaks are lo-
cated at approximately 711 and 725 eV, respectively, typical of
oxidised Fe3+ species (Figure 6, top left).[23,26] The peak at BE=
711 eV is traditionally assigned in the literature to Fe3+ species,
whereas the shoulder at approximately BE=719 eV belongs to
Both methodologies were effective in the incorporation of
Fe quantities close to the theoretical loadings (0.5–0.7 wt%
actual loading for 0.5 wt% Fe materials and 1.8 wt% for 1 wt%
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Figure 6. Representative XPS spectra of Fe-containing Z40c and Z15c materi-
als in the region of the Fe2p3/2 peak. For Fe 1/Z40c-H (BM), the whole Fe2p
XPS spectrum is shown, which includes the measured data (black points).
BEs [eV] were corrected with respect to the C1s peak (284.6 eV).
similar profiles irrespective of the Fe loading (0.5 or 1 wt% Fe),
the methodology employed for the preparation of the materi-
als and even after several reuses in the selected alkylation pro-
cess, in good agreement with supported iron oxide nanoparti-
cle materials reported previously.[23,26]
Figure 4. SEM micrographs of: (top) Z40c-H, (middle) Fe 0.5/Z40c-H (MW)
and (bottom) reused Fe 1/Z40c-H BM (after seven catalytic cycles).
The results of the surface acidity characterisation of the ma-
terials are included in Table 2 and Figure 7. The parent calcined
zeolites possess typical acidities reported previously for ZSM-5
materials, with a significant contribution of Lewis and Brønsted
(major) acid sites.[25,27–30] Interestingly, the different treatments
of the parent ZSM-5 gave rise to materials with essentially dif-
ferent surface acid properties although no clear correlation be-
tween acidity and textural properties could be made (see
Table 1 and Table 2). The development of mesoporosity in
ZSM-5 improved the accessibility of 2,6-dimethylpyridine
(DMPY) to Brønsted acid sites compared to the parent zeolites,
in good agreement with previous results.[25,28–30] However, in
contrast to previous studies, desilication of the parent zeolites
by NaOH treatment (Z40c-XNaOH and Z15c-YNaOH samples)
resulted in significant pore blockage as demonstrated by the
N2 physisorption studies, which hindered the accessibility of
probe molecules (even pyridine; PY) to acid sites significantly
(Table 1 and Table 2). The incorporation of iron oxide nanopar-
ticles in alkali-treated materials by the BM methodology gave
materials with virtually no acidity (with the exception of Fe
0.5/Z40c-0.4NaOH-BM), in agreement with the negligible acces-
sible surface (Table 2). The MW deposition of Fe onto alkali-
treated materials was only attempted for Z15c-0.8NaOH and
led to an essentially similar low-acidity material that was not
further investigated (not shown).
Figure 5. TEM micrographs of: (top) Fe 0.5/Z40c-H (MW), scale bar=10 nm,
(bottom left) Fe 1/Z40c-H (BM), scale bar=20 nm, and (bottom right) reused
Fe 1/Z40c-H (BM) (seven catalytic cycles), scale bar=10 nm.
a satellite peak, characteristic of Fe3+ species seen in Fe2O3
materials.[23,26]
Therefore, the XPS results confirmed the presence of fully
oxidised Fe species (mostly Fe3+ species, hematite phase) with
The accessibility of probe molecules increased remarkably
after the acid-washing treatment due to their excellent textural
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conversion in the proposed alkylation reaction was obtained
for calcined microporous ZSM-5 materials regardless of their
Si/Al ratios and despite their high total and Lewis acidities
(Tables 2 and 3). The negligible conversion to product for the
microporous materials is related to their small pores that may
be able to accommodate the starting materials (toluene and
benzyl chloride) but not the alkylated ortho-, meta- and even
para-diphenylmethane, which most probably remain inside the
zeolite pores. These results are in good agreement with litera-
ture reports for similar systems.[19,27,31]
Table 2. Surface acidity (measured using PY and DMPY as probe mole-
cules at 2008C) of the different catalysts synthesised in this work.
Catalyst
PY
DMPY
[mmolgÀ1
(B)
Lewis acidity
[mmolgÀ1
(B+L)
]
]
[mmolgÀ1
]
(L)
Z40c
218
200
309
529
470
347
30
<10
145
<10
25
14
51
204
149
135
244
271
219
24
<10
113
<10
18
28
15
53
89
72
12
42
35
Fe 0.5/Z40c (MW)
Fe 0.5/Z40c (BM)
Z15c
174
285
199
128
6
–
32
–
7
Fe 0.5/Z15c (MW)
Fe 0.5/Z15c (BM)
Z40c-0.4NaOH
Z40c-0.8NaOH
Fe 0.5/Z40c-0.4NaOH (BM)
Z15c-0.4NaOH
Z15c-0.8NaOH
Fe 0.5/Z15c-0.8NaOH (BM)
Z40c-H
Fe 0.5/Z40c-H (MW)
Fe 0.5/Z40c-H (BM)
Fe 1/Z40c-H (BM)
Z15c-H
Gratifyingly, the incorporation of iron oxide nanoparticles
into microporous Z40c and Z15c [Fe0.5 and Fe1/Z40c (MW)
and (BM)] provided quantitative conversion to alkylated prod-
ucts under identical reaction conditions regardless of the
methodology utilised for Fe deposition (MW or BM; Table 3).
Such excellent activities were observed for Fe-containing mate-
rials, which generally exhibited reduced Lewis (and total) acidi-
ties compared with parent Z40c and Z15c (Figure 7).
These important differences can be correlated to the devel-
opment of highly active and accessible Lewis acid sites on the
external surface of the zeolites, which are able to catalyse the
alkylation reaction efficiently. Similar catalysts were prepared
on mesoporous Si-MCM-41 and MSU-X silicas, that is, with
a 0.5 wt% Fe loading and using both MW and BM techniques.
In both cases, the catalysts were not active for this reaction,
which thus supports the role of
30
2
264
291
240
322
367
362
329
249
238
151
250
355
320
294
Fe 0.5/Z15c-H(MW)
Fe 0.5/Z15c-H (BM)
Lewis acidity was calculated as the difference between the PY and DMPY
values by assuming that all DMPY titrates Brønsted sites (B) selectively
and PY titrates both Brønsted and Lewis acid sites in the materials (B+L,
see Experimental Section).
acidity on the catalytic activity of
these samples. The presence of
nanoparticles deposited prefer-
entially on the external surface
(BM)[23b,32] as well as a contribu-
tion within the pores of porous
materials (MW) has been demon-
strated previously.[23] The incor-
poration of Fe also influences
the Al environment critically
through an observed synergetic
effect between Al and Fe.[23a]
Figure 7. Comparison of acidity [mmolgÀ1] versus catalytic conversion [%] for (Fe)Z40c and (Fe)Z15c. Reaction con-
Alkali treatment of Z40c and
Z15c rendered pore-blocked ma-
terials with significantly reduced
ditions: 2 mL toluene, 0.2 mL benzyl chloride, 0.025 g catalyst, microwave irradiation, 300 W, 5 min.
properties (Table 2). Interestingly, subsequent HCl washing of
both Z40c-NaOH and Z15c-NaOH materials generated almost
exclusively Brønsted acid sites, which were only slightly in-
creased (together with Lewis acidity) in Fe-containing materials
(Table 2, Z-NaOH-HCl and Fe 0.5/Z-NaOH-HCl). In this particular
case, the Fe incorporation methodology (MW or BM) did not
have a significant effect on the acidity of the systems.
textural properties and total acidity (Brønsted+Lewis), which
are catalytically inactive in the alkylation reaction (Table 3). Un-
expectedly, the incorporation of Fe into Z40c and Z15c-NaOH
materials rendered materials with an inferior total (and Lewis)
acidity, which were completely inactive in the investigated
Lewis-acid-catalysed alkylation regardless of the deposition ap-
proach (MW or BM; Table 3). These results were rather unex-
pected so further studies were performed to gain a better un-
derstanding of the synthesised materials.
Upon full characterisation, the catalytic activity of all syn-
thesised materials was investigated in the alkylation of toluene
with benzyl chloride as a model reaction promoted by Lewis
acidity.[22] The selected model reaction provided interesting in-
sights into the location of the active sites (Lewis and Brønsted)
as well as the effect of the generated porosity/treatment for
ZSM-5 materials. The results included in Table 3 show that
blank runs (in the absence of catalyst) provided negligible con-
version to products in the systems as expected. Similarly, no
The significantly different XRD patterns of BM-incorporated
Fe 0.5/Z15c-NaOH and Fe 0.5/Z15c-H materials are compared
in Figure 8. Several crystalline diffraction lines were observed
at 2q=23.3, 23.5, 24.5, 30.5 and 45.48 in the pattern of Fe 0.5/
Z15c-H (blue line, Figure 8). These correspond to an aluminium
silicate ZSM-5 structure.[7,20] No lines that correspond to iron
oxide nanoparticles could be visualised in this material be-
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tion of alumina debris formed upon alkali treatment
with the Fe precursor under BM conditions (reactive
milling).
Table 3. Catalytic activity of hierarchical zeolite materials in the microwave-assisted al-
kylation of toluene with benzyl chloride catalysed by Lewis acids.
The HCl treatment of Z40c and Z15c-NaOH materi-
als generally improved both the total and particularly
the Brønsted acidity in the final hierarchically porous
Z40c-H and Z15c-H materials, which exhibited signifi-
cantly improved and more accessible acid sites
(Table 2). However, these materials possessed essen-
tially Brønsted acid sites hence exhibited no conver-
sion in the toluene alkylation with benzyl chloride
(Table 3 and Figure 9). Lewis acid sites provided by
the extra-framework Al phases in calcined zeolites
were generally found to be inactive in the tested
reaction.
Catalyst
Conv. [%]
Total
Select. [%]
ortho
meta
para
Blank (no catalyst)
Z40c
Fe 0.5/Z40c (BM)
Fe 0.5/Z40c (MW)
Z15c
Fe 0.5/Z15c (BM)
Fe 0.5/Z15c (MW)
Z40c-NaOH
Fe 0.5/Z40c-NaOH (BM)
Z15c-NaOH
–
<5
>99
>99
<5
>99
>99
–
<10
<5
–
–
>99
87
–
11
8
–
40
43
43
47
36
35
–
–
48
49
49
38
49
47
–
8
15
15
18
–
8
-
–
–
10
7
10
42
41
–
51
59
–
Upon Fe incorporation, Lewis acidity is created in
the materials from the deposition of iron oxide nano-
particles that lead to highly active and accessible
active sites on the surface of the hierarchical zeolites,
which are able to catalyse the alkylation reaction effi-
ciently to give high yields (>90%; Table 3 and
Figure 9). Interestingly, the Fe 0.5/Z40c-H and Fe 0.5/
Z15c-H materials exhibited very similar acid proper-
ties to those of the parent Z40c-H and Z15c-H mate-
rials, with the exception of the generated Lewis acidi-
ty upon the incorporation of iron oxide nanoparticle
(Figure 9). Product selectivity was similar for all Fe-
containing catalytically active materials with a slightly
Fe 0.5/Z15c-NaOH (BM)
Z40c-H
–
–
Fe 0.5/Z40c-H (BM)
Fe1/Z40c-H (BM)
reused Fe1/Z40c-H (BM)
(seven catalytic uses)
Fe/Z40c-H (MW)
Z15c-H
39
43
45
51
50
45
>99
97
–
>99
97
8
–
10
12
44
–
44
40
48
–
46
48
Fe 0.5/Z15c-H (BM)
Fe 0.5/Z15c-H (MW)
Reaction conditions: 2 mL toluene, 0.2 mL benzyl chloride, 0.025 g catalyst, microwave
irradiation, 300 W, 5 min.
cause of the low Fe loading, but the presence of such Fe2O3
species was confirmed by XPS (Figure 6). However, most of
these reflections were not present in the XRD pattern of Fe
0.5/Z15c-NaOH (green line, Figure 8), which exhibited a broad
amorphous band centred at 2q=258 and various intense dif-
fraction lines that correspond to the formation of an iron alu-
minate phase (2q=31.8, 37.5—maximum intensity that corre-
sponds to the (311) diffraction line—and 44.68),[33,34] with no
evidence of the distinctive lines of iron oxide nanoparticles.
The formation of the iron aluminate phase has been reported
previously to be generated from Fe-Al2O3 composites under
similar ball-milling metallurgy processes from reactive sintering
between Fe and Al2O3.[33,34]
Figure 8. Comparison of the XRD patterns of Fe 0.5/Z15c-H (blue line) and
Fe 0.5/Z15c-NaOH (green line). The XRD patterns show the presence of an
FeAl2O4 phase, with its distinctive (311) diffraction line at 2q=37.58.[33,34]
Iron aluminate does not have any acidity, which can explain
the observed reduced acidity upon Fe incorporation in these
materials, and consequently,
such a material is not expected
to have any catalytic activity in
the alkylation reaction.[33,34] To
further support this theory, iron
aluminate prepared following
a literature method[34] was also
tested in the alkylation and
showed no conversion to prod-
ucts under the investigated con-
ditions. The formation of this
Figure 9. Comparison of acidity [mmolgÀ1] versus catalytic conversion [%] for (Fe)Z40c-H and (Fe)Z15c-H. Reaction
inert inorganic iron aluminate
phase was induced by the reac- conditions: 2 mL toluene, 0.2 mL benzyl chloride, 0.025 g catalyst, microwave irradiation, 300 W, 5 min.
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preferential selectivity to the para derivative as expected (less
sterically hindered product; Table 3). The observed differences
between inactive iron oxide nanoparticles on desilicated
NaOH-treated zeolites and their hierarchical counterparts is be-
lieved to be related to Fe-Al synergy reported previously[23,35,36]
in this case with the acidity of the zeolite support. In the case
of Fe-containing desilicated zeolites, both porosity and acidity
are blocked (Figure 10). Fe incorporation leads to iron oxide
nanoparticle species of relatively low acidity [Table 2; Fe 0.5/
Z40c-0.4NaOH (BM)] that are poorly active and unable to pro-
mote the reaction to a large extent. Upon acid washing, both
the porosity and acidity influence an improved interaction be-
tween Fe-Al to bring back the synergistic role of acidity and
thus lead to highly active materials. This is also the case for Fe-
containing microporous zeolites (Table 3 and Figure 10) in
which Fe-Al synergism provides high activities in the alkylation
of toluene with benzyl chloride.
oxide species into the zeolite micropores upon Fe incorpora-
tion; Table 1) might actually be significant if we take into ac-
count the differences in Lewis acidity between the two materi-
als (219 mmolgÀ1 for microporous vs. 35 mmolgÀ1 for Fe-con-
taining mesoporous Z15c-H; Table 2). The explanation for this
effect is not trivial and will require further investigations, par-
ticularly as the proposed assumption for Lewis acidity may not
be correct for the hierarchical zeolites. In any case, current
findings (if we leave aside the consideration of Lewis acidity)
seem to indicate that the developed mesoporosity in the ma-
terials does not add significant advantages in terms of catalytic
activity for the particular alkylation reaction selected.
The findings obtained for the different systems and plausible
explanations for the different Fe-containing materials are sum-
marised and depicted in Figure 10. The synthesised materials
comprise a) active iron oxide nanoparticles that are incorporat-
ed predominantly on the external surface of the untreated
ZSM-5 zeolites by BM or MW (Fe/Z40c and Fe/Z15c), b) an iron
aluminate phase from the reactive milling of Fe and extra-
framework alumina species upon desilication in alkali-treated
Z40c and Z15c-NaOH materials and c) a combination of highly
active iron oxide nanoparticles on the external surface (plus
some contribution within the pores) of acid-reconstituted
ZSM-5 hierarchical Fe/Z40c-H and Fe/Z15c materials.
The reusability of the Fe-containing materials was tested in
the proposed alkylation reaction. The SEM and TEM analysis,
textural properties and Fe content (Table 2 and Figures 3 and
5) support the stability of the Fe-incorporated hierarchical ma-
terials, the activities of which were almost unchanged after
seven catalytic cycles (see Table 3). Regeneration by calcination
at 4008C for 2 h was conducted after the first run (a significant
loss of activity was observed, up to 70% of the initial conver-
sion) to preserve such activity. Upon calcination after the first
reuse, Fe1/Z40c-H (BM) provided remarkable catalytic activity,
even slightly superior to that in the first catalytic run, after sub-
sequent runs with up to six reuses (Table 3). The conditioning
of the catalyst after every reuse (after the first calcination)
comprised a simple washing step with ethanol and acetone
and drying overnight at 1108C before reuse in the alkylation
reaction.
Figure 10. Schematic representation of the synthesis and proposed struc-
tures of Fe/Z40c and Fe/Z15c (top), Fe/Z40c-NaOH and Fe/Z15c-NaOH
(middle) and Fe/Z40c-H and Fe/Z15c-H (bottom).
A hot filtration test was conducted half-way through the re-
action (typically after 3 min reaction at ꢀ45–55% conversion)
to ascertain the presence of leached Fe species in the solution.
Experiments were performed similarly to all catalytic reactions
(Table 3), and the catalyst was removed by filtration quickly im-
mediately after the microwave was stopped. The catalyst was
put into a clean vial with fresh substrates (benzyl chloride and
toluene) and subjected to another reaction to provide essen-
tially identical yields of diphenylmethanes compared to the
conventional reactions.
A strict catalytic activity comparison between parent versus
hierarchical zeolites was conducted to prove any advantages
of the developed mesoporosity for the selected process. With
this purpose, 1 and 3 min microwave experiments were per-
formed for both Fe 0.5/Z15c-H (BM) and Fe 0.5/Z15c (BM) as
well as for MW-prepared materials. The results (not shown) in-
dicated only a slightly superior conversion for hierarchical Fe-
containing Z15c-H materials compared to the parent micropo-
rous counterparts [that is, 52% conversion for Fe 0.5/Z15c-H
(BM) vs. 43% conversion for Fe 0.5/Z15c (BM), microwave
300 W, 3 min reaction under identical conditions to those
stated in Table 3]. The apparent slight difference in activity
(which could be attributed to the observed migration of iron
Importantly, after the catalyst was removed, the reaction fil-
trate was further reacted for 60 min under microwave irradia-
tion. A negligible increase in conversion was observed in re-
peated runs (i.e., from 55% conversion in the original filtrate
to 53–60% conversion after 60 min), which supports the truly
heterogeneous nature of the catalytic system. Furthermore, in-
ductively coupled plasma mass spectrometry (ICP-MS) of the
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Subsequently, the alkaline-treated samples were washed with HCl
(0.1m) to remove amorphous Al-rich debris. Each sample (1 g;
Z40c-XNaOH or Z15c-YNaOH) was stirred in aqueous HCl (100 mL;
0.1m) for 6 h at 658C. The solid was collected by filtration and
washed with distilled water. Treatment yields varied from 14–20%.
Hierarchical ZSM-5 materials after acid washing are denoted as
Z40c-XNaOH-HCl and Z15c-YNaOH-HCl. For brevity, two samples,
Z40c-0.4NaOH-HCl and Z15c-0.8NaOH-HCl, are denoted Z40c-H
and Z15c-H, respectively, in the manuscript. Z40c-H denotes a hier-
archical ZSM-5 porous zeolite calcined with an initial Si/Al molar
ratio of 40 obtained after treatment with a 0.4m NaOH solution
and washed with a 0.1m solution of aqueous HCl. Similarly, Z15c-H
corresponds to a hierarchical ZSM-5 porous zeolite calcined with
an initial Si/Al 15 ratio obtained after washing with a 0.8m NaOH
solution (optimum conditions) and reconstituted with a 0.1m solu-
tion of aqueous HCl. The NaOH concentrations were selected to
provide good textural and acid properties in the final materials.
filtrate after 60 min under microwave irradiation showed no
detectable quantities of Fe in the solution (<0.5 ppm), in good
agreement with the stability of similar Fe-containing materials
demonstrated previously.[36,37] These results were again in good
agreement with the XPS data for the Fe content of Fe1/Z40c-H
(BM) after seven uses.
Conclusion
The alkylation of toluene with benzyl chloride was selected to
demonstrate the inherent advantages of nanoparticle-support-
ed systems prepared under a new mechanochemical deposi-
tion method as well as a microwave-assisted protocol, which
allow the design of stable and active zeolites that contain iron
oxide nanoparticles for a series of useful catalytic transforma-
tions.[35,36] An interesting synergetic effect between Fe and Al
in the zeolites was observed for the synthesised materials in
the alkylation of toluene with benzyl chloride, in which the
Lewis acid sites of extra-framework Al phases in calcined zeo-
lites are generally inactive. The Fe-containing zeolites were
also remarkably stable under the investigated conditions and
were reusable up to seven times with extremely stable Fe spe-
cies in their structure (no Fe leaching and no significant
changes in Fe species were observed upon reuse).
Synthesis of nanoparticle-functionalised mesoporous
zeolites
Microwave-assisted deposition of Fe-containing materials
Materials were prepared according to a methodology reported pre-
viously.[23a] Typically, zeolite (0.5 g), ethanol/water (1:2, 2.0 mL) and
the target quantity of FeCl2·6H2O to reach a theoretical 0.5 wt% Fe
content in the material were microwaved by using a CEM-DISCOV-
ER reactor for 15 min at 300 W (maximum power output), which
led to a maximum temperature of 100–1108C (power-control
method). The solids were collected by filtration and washed thor-
oughly with ethanol and acetone, oven dried overnight at 1008C
and calcined in air at 4008C for 2 h to stabilise iron oxide nanopar-
ticles on the zeolites before their use in the alkylation reaction.[35]
Materials are denoted as Fe/Z (MW) derived materials in which Fe
represents a theoretical Fe incorporation of 0.5 wt% in the final
material, Z refers to the zeolitic material synthesised, and MW indi-
cates the microwave-assisted incorporation of the metal.
The Fe-containing materials prepared in line with previous
reports proved that the generation of iron oxide nanoparticles
on mesoporous aluminosilicate supports can enhance the cata-
lytic activity of the final catalysts significantly.[23a,35–37]
Experimental Section
Materials and methods
Two commercial MFI zeolites, CBV8014 (Zeolyst International, Si/Al
molar ratio=40) and CBV3024E (Zeolyst International, Si/Al molar
ratio=15), were used as starting materials after calcination at
5508C for 5 h in air (rate of 1008C hÀ1). These are denoted as Z40c
and Z15c, in which Z stands for the type of zeolite (ZSM-5), the
number (40 or 15) refers to the Si/Al ratio according to the manu-
facturer’s specifications, and the letter c refers to the calcined
material.
Mechanochemical preparation of Fe-containing materials
Materials were also prepared using an innovative mechanochemi-
cal protocol developed recently in our group.[23b] Typically, the pre-
formed ZSM-5 material (2 g) was ground with the target quantity
of metal precursor (FeCl2·6H2O, Sigma–Aldrich) in the solid phase
to achieve a 0.5 wt% Fe loading in the final material (unless other-
wise stated). The mechanochemical protocol was performed by
using a planetary ball mill (Retsch PM 100 model) under conditions
optimised previously (10 min milling at 350 rpm). The obtained
materials were calcined in air at 4008C for 2 h. Materials are denot-
ed as Fe 0.5 or Fe 1/Z (BM), in which Fe 0.5 or Fe 1 stand for a theo-
retical Fe incorporation of 0.5 or 1 wt%, respectively, in the final
material (unless otherwise stated), Z refers to the zeolitic material
synthesised, and BM stands for ball-milling incorporation of the
metal oxide nanoparticles.[23b]
Post-synthetic treatments
Post-synthetic desilication treatment was conducted following the
methodology reported by Groen et al.[18,24] in which zeolites were
treated with different concentrations of NaOH solution (0.4 and
0.8m) and washed subsequently with HCl to obtain hierarchical
ZSM-5 porous zeolites.
Typically, the parent zeolite (3.3 g; Z40c or Z15c) was stirred mag-
netically at 600 rpm and 658C in an aqueous solution of NaOH
(100 mL, 0.4 or 0.8m) for 30 min. The material was collected by fil-
tration and washed thoroughly with distilled water. Alkali-treated
materials are denoted as Z40c-XNaOH and Z15c-YNaOH, in which
Z stands for the type of zeolite (ZSM-5), the number (40 or 15)
refers to the Si/Al molar ratio according to the manufacturer’s
specifications, the letter c refers to the calcined material and X or Y
refers to the concentration of alkali utilised to treat the samples
(0.4 and 0.8m NaOH).
Characterisation
The porosity of the materials was measured by N2 adsorption at
77 K by using an AUTOSORB-6 apparatus. Samples were degassed
for 5 h at 373 K at 5.10À5 bar before measurement. The BET surface
area was estimated by using the multipoint BET method and the
adsorption data in the relative pressure (P/P0) range of 0.05–0.30.
The pore size distribution was calculated from the adsorption
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branch of the N2 isotherms using the Barret–Joyner–Halenda (BJH)
method. The mesoporous volume was calculated from the cumula-
tive pore volume distribution curve. The micropore volume was
calculated by the t-plot method.
ing under projects P10-FQM-6711 (Consejeria de Ciencia e Innova-
cion, Junta de Andalucia) and CTQ2011 28954-C02-02 (MICINN).
Both R.L. and J.G.M. acknowledge support from MICINN under
cooperation project CTQ2011-28954. J.G.M. gratefully acknowl-
edges funding under the WAVES project (Spanish MINECO and
UE, ERA-NET CAPITA).
The morphology and structure of the materials was investigated
by TEM and SEM. TEM micrographs were obtained by using a JEM-
2010 microscope (JEOL, 200 kV, 0.14 nm resolution). Samples were
prepared by dripping a sonicated suspension of the materials in
ethanol on a carbon-coated copper grid. SEM analysis was per-
formed on gold-coated samples by using a JSM-840 microscope
(JEOL).
Keywords: alkylation · mesoporous materials · nanoparticles ·
supported catalysts · zeolites
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Acknowledgements
R.L. gratefully acknowledges support from the Spanish MICINN by
the concession of a RyC contract (ref. RYC-2009-04199) and fund-
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Received: July 19, 2014
Revised: August 12, 2014
Published online on && &&, 0000
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Iron out the creases: Supported iron
oxide nanoparticles are incorporated on
hierarchical zeolites by microwave-
assisted impregnation and mechano-
chemical grinding. The catalytic activi-
ties of the synthesized nanomaterials
are investigated in the alkylation of tol-
uene with benzyl chloride under micro-
wave irradiation.
A. Grau-Atienza, R. Campos, E. Serrano,
M. Ojeda, A. A. Romero,
J. Garcia-Martinez,* R. Luque*
&& – &&
Insights into the Active Species of
Nanoparticle-Functionalized
Hierarchical Zeolites in Alkylation
Reactions
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