Acidity versus metal-induced Lewis acidity in zeolites for Friedel–Crafts acylation

Article abstract of DOI:10.1016/j.crci.2016.03.008

Acid catalysts including Ni, Ag and Fe-loaded zeolites of different structures were prepared either via cationic exchange or impregnation techniques from pristine H-zeolites (BEA, and MFI). Their catalytic activity was evaluated in the liquid-phase Friedel–Crafts acylation of anisole with propanoic acid. It turned out that, whatever the doping procedure was, the zeolite loaded with transition metals led to considerable decrease in propanoic acid conversion, regardless of the nature or the metal content. However, the extent of this detrimental effect followed the order: Ag⁺?>?Ni²⁺?>?Fe³⁺. Pristine acidic zeolites were not only found to be the most active, but also to be the most selective toward ortho- and para-acylation products. H-ZSM-5 zeolites yielded the highest intrinsic activity, with TOF values of 0.09?h^?1. The catalyst activity proved to be essentially attributed to the density and accessibility of Br?nsted acid sites, playing a key role in the activation of the reactants. Br?nsted sites are proposed to be the most likely catalytic species for performing this Friedel–Crafts acylation.

Full text of DOI:10.1016/j.crci.2016.03.008

C. R. Chimie xxx (2016) 1e10
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ꢀ
Full paper/Memoire
Acidity versus metal-induced Lewis acidity in zeolites for
FriedeleCrafts acylation
,
Claire Bernardon a, Manel Ben Osman a *, Guillaume Laugel b, Benoît Louis a,
Patrick Pale a
a
ꢁ
ꢀ
ꢀ
Laboratoire de Synthese et Reactivite Organiques et Catalyse, Institut de Chimie, UMR 7177, 4, rue Blaise-Pascal,
67000 Strasbourg, France
b
ꢀ
ꢀ
ꢀ
ꢀ
Sorbonne Universites, UPMC Univ Paris 06, UMR 7197, Laboratoire de Reactivite de Surface, 3, rue Galilee,
94200 Ivry-sur-Seine, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Acid catalysts including Ni, Ag and Fe-loaded zeolites of different structures were prepared
either via cationic exchange or impregnation techniques from pristine H-zeolites (BEA, and
MFI). Their catalytic activity was evaluated in the liquid-phase FriedeleCrafts acylation of
anisole with propanoic acid. It turned out that, whatever the doping procedure was, the
zeolite loaded with transition metals led to considerable decrease in propanoic acid con-
version, regardless of the nature or the metal content. However, the extent of this detri-
Received 10 November 2015
Accepted 10 March 2016
Available online xxxx
Keywords:
FriedeleCrafts acylation
Metal-doped zeolites
Lewis acidity
mental effect followed the order: Agþ > Ni2þ > Fe3þ
.
Pristine acidic zeolites were not only found to be the most active, but also to be the most
selective toward ortho- and para-acylation products. H-ZSM-5 zeolites yielded the highest
intrinsic activity, with TOF values of 0.09 hꢀ1. The catalyst activity proved to be essentially
attributed to the density and accessibility of Brønsted acid sites, playing a key role in the
activation of the reactants. Brønsted sites are proposed to be the most likely catalytic
species for performing this FriedeleCrafts acylation.
Brønsted acidity
ꢀ
© 2016 Academie des sciences. Published by Elsevier Masson SAS. This is an open access
article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/
4.0/).
r é s u m é
ꢀ
ꢀ ꢀ
ꢀ
ꢀ
Des cations metalliques (Ni, Ag et Fe) ont ete incorpores au sein de zeolithes BEA et ZSM-5
ꢀ
ꢀ
par des techniques d'echange cationique et d'impregnation afin d'obtenir de nouveaux
ꢀ
ꢀ ꢀ
ꢀ
ꢀ
catalyseurs acides solides. Ces materiaux ont ensuite ete testes dans une reaction
d'acylation de FriedeleCrafts entre l'anisole et l'acide propanoïque en phase liquide. Il s'est
ꢀ ꢀ
ꢀ
ꢀ
avere que l'ajout de metaux de transition au sein de la structure zeolithique entraîne une
ꢀ
ꢀ
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diminution considerable de la conversion, independamment de la nature ou de la quantite
ꢀ
ꢀ
de metal introduite. Dans des conditions catalytiques similaires, les zeolithes parentes
ꢀ
ꢀ
ꢁ
presentent une meilleure activite intrinseque (TOF), en particulier H-ZSM-5, avec une
ꢀ
ꢀ
production plus elevee de produits d'acylation (ortho- et para-), surpassant ainsi les per-
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
formances catalytiques des zeolithes dopees par des metaux. L'accessibilite et la densite de
ꢀ ꢀ
ꢀ
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sites acides de Brønsted se sont averees determinantes pour ce type de reaction.
ꢀ
© 2016 Academie des sciences. Published by Elsevier Masson SAS. This is an open access
article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/
4.0/).
* Corresponding author.
E-mail address: benosman@unistra.fr (M. Ben Osman).
http://dx.doi.org/10.1016/j.crci.2016.03.008
ꢀ
1631-0748/© 2016 Academie des sciences. Published by Elsevier Masson SAS. This is an open access article under the CC BY-NC-ND license (http://
creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: C. Bernardon, et al., Acidity versus metal-induced Lewis acidity in zeolites for FriedeleCrafts
acylation, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.03.008

2
C. Bernardon et al. / C. R. Chimie xxx (2016) 1e10
(Zeolyst CBV500) and one home-made H-ZSM-5 [18], were
1. Introduction
activated under air atmosphere at 823 K for 12 h before use.
Commercial H4SiW12O40 heteropolyacid (Aldrich) and two
sulphated oxides (SnO2 and ZrO2/SO42ꢀ) were used as
received.
FriedeleCrafts reactions are among the most important
CeC bond-forming transformations in organic chemistry,
which haveled to awide rangeof applications. Among them,
FriedeleCrafts acylation provides one of the most simple
and powerful industrial routes to prepare aromatic ketones
in high yields [1]. The so-formed aromatic compounds
constitute valuable building blocks for fine chemistry,
pharmaceutical, cosmetic and agrochemical industries, as
well as for total synthesis of natural products [2,3].
The classical FriedeleCrafts acylation relies on homo-
geneous conditions in the presence of various Brønsted or
Lewis acids. For the former, sulfuric acid, hydrofluoric acid
or superacids are commonly used whilst for the later salts
such as ZnCl2, AlCl3, FeCl3, SnCl4, and TiCl4 have been used,
often in super-stoichiometric amounts. More recently, rare-
earth metal Lewis acids have been developed as efficient
and true catalysts [4]. However, these reactions suffer from
major drawbacks and disadvantages, due to the implication
of strong acids and the requirement of stoichiometric or
over-stoichiometric amounts of Lewis or Brønsted acids,
with all the difficulties to recover and isolate the products
from them, as well as the generation of large amounts of
salty and acidic wastes.
On one hand, one way to minimize some of these
problems is to perform FriedeleCrafts acylation directly
with carboxylic acids as acylating agents, instead of their
anhydrides or acyl chlorides, suppressing one step and the
waste associated with it. On the other hand, a wide variety
of heterogeneous acids have been explored as promoters in
order to overcome those aforementioned environmental
issues. Zeolites [5e7], clays [5], heteropolyacids [8,9],
nafion [10], P2O5/SiO2 [11] and solid “superacids”, i.e.
sulfate-supported metal oxides [12] and tungsto- or
molybdophosphoric acids supported on [Zr, Ti, Sn] oxides
[13], have been extensively studied in the general context
of FriedeleCrafts reactions. Less extensive investigations
have been performed for FriedeleCrafts acylations based
on carboxylic acids as acylating agents. Here also, rare-
earth metal salts proved to be efficient and true catalysts
[14]. Interestingly, some acidic and modified zeolites have
also been investigated in few cases [15,16]. Nevertheless,
the combination of Lewis acid metal cations and zeolites
has only been scarcely investigated in this context [17].
In the present contribution, we thus present pre-
liminary results about the role of zeolites having different
pore topologies, loaded with different metal cations at
various loadings as catalysts for the FriedeleCrafts acyla-
tion of anisole with propanoic acid. The influence of the
dealumination process was also briefly investigated.
Furthermore, the performances of these zeolite catalysts
have been compared with other solid acids, a hetero-
polyacid and two sulfated metal oxides (SnO2, and ZrO2).
2.2. Post-modifications of parent zeolites
2.2.1. Procedures for extra-framework aluminium (EFAl)
exaltation and removal
In order to create extra-framework aluminium (EFAl)
species, H-ZSM-5 and H-BEA were treated under mild
steaming conditions: hydrothermal treatment was per-
formed under nitrogen flow at 823 K for 24 h with water
pre-heated at 323 K. This procedure is known to remove
aluminium atoms from the framework [19]. To fully remove
these EFAl species from the channels, a second treatment of
these zeolites was performed by using Na2H2EDTA solution
(EDTA ¼ ethylenediaminetetraacetate) as described by T.
Xu et al. [20].
2.2.2. Metal-loaded zeolites
Transition metals namely nickel, silver and iron, were
introduced into pristine ZSM-5 and BEA zeolites (defined as
parent) according to different procedures. Classical ion
exchange in an aqueous phase and impregnation tech-
niques were applied. These metal doping strategies are
summarized hereunder:
Fe-loaded zeolites: iron was introduced into H-BEApar-
zeolites by an incipient wetness impregnation tech-
ent-2
nique using Fe(NO3)3$9H2O solution at two different metal
loadings: 0.25 and 10 wt%.
Ni-loaded zeolites: a dry impregnation procedure was
performed on H-ZSM-5parent-1 and H-BEAparent-1with nickel
carboxylate as a precursor in toluene as described else-
where [18]. The procedure was carried out in a rotary
evaporator for 30 min at ambient temperature. The catalyst
was dried for 1 h under vacuum at 393 K and then calcined
in air at 873 K for 3 h.
Ag-loaded zeolites: silver was loaded in the ZSM-5parent-2
zeolite as follows: the H-zeolite (2 g) was exchanged with a
0.1 M aqueous AgNO3 solution (50 mL). The resulting white
suspension was vigorously stirred overnight at 353 K in the
absence of light. After the first cationic exchange, the so-
lution was cooled, filtered through a nylon membrane
(0.2 mm), washed with distilled water and dried in an oven
for 24 h at 383 K. The resulting white powder was then
submitted to a second ion exchange by following the
aforementioned procedure. After drying for 24 h at 383 K,
the solid was finally calcined at 823 K for 12 h under air to
hinder silver reduction.
The as-obtained catalyst samples were referred to x%M-
ZSM-5 and y%M-BEA zeolites (M corresponds to the metal
introduced and x and y% to the amount initially used).
2. Experimental
2.3. Characterisation of the catalysts
2.1. Pristine solid acid catalysts
Although detailed characterisation of the zeolites was
not the aim of the present study, the quantity of metal (wt
%) and Si/Al ratios (SAR) were determined by XRF analyses.
Specific surface areas were determined by N2
Commercial zeolites, H-ZSM-5 (Zeolyst CBV5524 and
CBV2314), H-BEA (Petrobras or Zeochem PB), H-USY
Please cite this article in press as: C. Bernardon, et al., Acidity versus metal-induced Lewis acidity in zeolites for FriedeleCrafts
acylation, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.03.008

C. Bernardon et al. / C. R. Chimie xxx (2016) 1e10
3
adsorptionedesorption isotherms following the BET
method. The morphology of the particles was provided by
Scanning Electron Microscopy (micrographs were acquired
on a JEOL FEG 6700F microscope working at a 9 kV accel-
erating voltage) and the Brønsted acid site quantification
was performed by H/D isotope exchange [21]. All data are
summarized in Table 1.
(ppm) on the delta scale. The solvent peak was used as a
reference value (for 1H NMR: CDCl3 ¼ 7.26 ppm). GCeMS
analyses were performed with a TSQ Quantum (Thermo
scientific) MS detector, equipped with a HP5-MS column
(30m ꢁ 0.25 mm ꢁ 0.1
mm).
3. Results and discussion
Besides, XPS measurements of metal-promoted catalysts
3.1. The reaction between anisole and propanoic acid
(1% Ni-ZSM-5parent-1, 4% Ni-ZSM-5parent-1, 10% Fe-BEAparent-2
,
and 5.3% Ag-ZSM-5parent-2) were carried out to understand
the bonding state of the different metals in the zeolite
framework. The X-ray Photoelectron Spectroscopy (XPS)
Under the reaction conditions, a mixture of compounds
was produced (Fig. 1). The expected ortho and para acylated
compounds 3 were the main reaction products. Besides, the
acylated phenols 4 were also isolated as well as the phenyl
propanoate 5.
measurements were performed with
analyzer (SES R4000, Gamma data Scienta, pass energy
100 eV). The un-monochromatized AlK X-ray source
a hemispherical
a
The latter was probably formed by anisole demethyla-
tion followed by acylation. Compounds 4 could thus be
produced either by Fries rearrangement of this ester or by
demethylation of compounds 3.
(1486.6 eV, 11 kV, 17 mA) without a charge neutralizer was
applied to generate core excitation. The system was cali-
brated accordingto ISO15472:2001. The energyresolutionof
the system, measured as a full-width at half-maximum
(FWHM) for the Ag 3d5/2 excitation line, was 0.9 eV. The
powder samples were pressed into indium foil and mounted
on a dedicated holder and then UHV evacuated. During the
measurements, the pressure in the analysis chamber was
roughly 10ꢀ9 mbar. The area of the sample analysis was
approximately 3 mm2. All binding energy (BE) values were
charge-corrected to the carbon C1s excitation set at 285.0 eV.
The Shirley-type background subtraction was used to the
spectra prior tothefittingprocedure, where Voigt line shape,
i.e. Gaussian/Lorentzian functional (70:30) was applied.
Concerning the distribution of the products, the main
products formed over all samples are 3, ortho and para,
regardless of the zeolites used. Ortho- and para-acylated
products were produced in a 2:1 ratio over most of the
solids, revealing that this reaction is not governed by
shape-selectivity. We have therefore decided to focus the
rest of this work on acylated products 3 selectivity. It is wise
to mention that the impact of the metal content on the
selectivity could not be easily discussed because of large
differences in the conversion levels.
The activity of the catalysts was measured after 24 h of
reaction and during this period no significant deactivation
phenomenon seemed to occur. Also, it is worth noting that
the leaching process was evaluated to assess if metallic
species were removed from the catalyst into the solution.
The reaction was stopped after 24 h and the catalyst was
removed by filtration. The recovered zeolite was calcined
under air and then reused under the same conditions and
the same PA conversion level could be achieved. Hence,
leaching of metallic active species was discarded, thus
indicating a true heterogeneous catalysis of metal-loaded
zeolite catalysts.
2.4. FriedeleCrafts acylation
The activity of the catalysts was evaluated in the reac-
tion between anisole and propanoic acid (Fig. 1) according
to the following protocol:
The solid acid (iso-mass conditions, 250 mg) was added
to a mixture of anisole (10 mmol, SigmaeAldrich) and
propanoic acid (2.5 mmol, SigmaeAldrich) in a closed
reactor system. Upon sealing, the tube was heated at 423 K
for 24 h. After this duration and cooling, the crude was
filtered on a Nylon Millipore membrane (0.2
mm) with
CDCl3 as a filtration solvent and analysed by 1H NMR and
GCeMS.
3.2. Screening of solid acids
The products were identified and characterised by 1H
NMR recorded on
300 MHz. Chemical shifts are given in parts per million
a
Bruker Avance Spectrometer at
Classical ion exchange methods allowed introducing
nickel, silver and iron into ZSM-5 and BEA zeolites (Table 1).
Table 1
Mnþ zeolite catalysts prepared by ion exchange and impregnation methods: Si/Al ratio, SSA values, number of Brønsted acid sites and metal amount (wt%).
Catalysts
Metal amount (wt% XRF)
Si/Al ratio
SSA (m2/g)
Acidity (mmol Hþ/g catalyst)
H-ZSM-5parent-1
e
12.5
e
392
332
319
425
e
1.58
1.43
1.24
1.48
1.60
1.16
2.35
1.33
1.63
1.40
0.85
1% Ni-ZSM-5parent-1
4% Ni-ZSM-5parent-1
H-ZSM-5parent-2
2.6% Ag-ZSM-5parent-2
5.3% Ag-ZSM-5parent-2
H-BEAparent-1
1%Ni-BEAparent-1
H-BEAparent-2
0.25% Fe-BEAparent-2
10% Fe-BEAparent-2
1
3.65
e
17.4
11.5
15.3
15.5
14.9
16.2
13.8
e
0.01
0.03
e
e
e
1
e
e
485
e
7.2
12.4
14.8
379
Please cite this article in press as: C. Bernardon, et al., Acidity versus metal-induced Lewis acidity in zeolites for FriedeleCrafts
acylation, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.03.008

4
C. Bernardon et al. / C. R. Chimie xxx (2016) 1e10
Fig. 1. FriedeleCrafts acylation of anisole with propanoic acid.
In these metal-loaded zeolites, SAR ratios slightly increased
compared to the parent zeolites, indicating that metal ions
probably exchanged some of EFAl species naturally present.
However, specific surface areas diminished after the addi-
tion of metal, whatever the type of the zeolite structure
(MFI or BEA), suggesting the presence of metal species in-
side the porous network. Likewise, the concentration of
Brønsted acid sites decreased while raising the metal
amount for doping the zeolites, thus confirming an ex-
change between metallic species and protons. XRF analysis
provided the amount of metal loaded within the zeolites
(Table 1). The collected values revealed that the metal
introduction was not proportional to the quantity initially
used. However, the metal introduction proved to be sen-
sitive to the metal nature as well as to the introduction
methodology [17].
It is also noteworthy that the Brønsted acid site con-
sumption, after the metal loading procedure, was more
pronounced with zeolites possessing the highest Brønsted
acid site density (BEA zeoliteparent-1 with 2.35 mmol/g). This
suggests that the acid site density and their proximity may
have an influence on the metal introduction into the zeolite
pores.
pristine zeolites (indeed, XRF analysis is unable to distin-
guish framework and extra-framework species).
To check the possible structure changes or collapse of
the zeolite framework during the post-synthesis modifi-
cations, XRD analysis was performed. The related XRD
patterns (not shown) confirmed the preservation of zeolite
structures in promoted samples, in agreement with earlier
reports [22,23]. The presence of metal oxides (possibly
formed after the calcination step) could be discarded based
on these XRD patterns. One may therefore expect that
metal cations are well dispersed within the zeolite and
probably present in a rather isolated form. It is important to
point out that the strength of metal coordination, locali-
zation and distribution among all the catalysts may be
different, all these features being directly related to the
catalytic behaviour. The obtained catalysts were applied to
the reaction described above (Fig. 1).
The solid acids have been evaluated under isomass
conditions in the FC reaction between anisole and prop-
anoic acid. According to Fig. 2, H-ZSM-5 and H-BEA zeolites
exhibited the highest propanoic acid conversion among all
solid acids. Sulfated metal oxides as well as amorphous
silica-alumina were barely active in this reaction, thus
indicating the importance of decent pore sizes as well as a
need for strong acidity. While the highest conversion was
achieved over H-ZSM-5, the highest selectivity in acylated
products 3 was achieved over the H-BEA zeolite (70%).
Hence, one may suggest that the catalytic reaction is
occurring on the external surface for ZSM-5 zeolite, whilst
The EDTA-treated ZSM-5 zeolite led to achieve a higher
SAR value, i.e. roughly 20 with respect to 12, before the
complexation
treatment.
Indeed,
extra-framework
aluminium species have been removed from the zeolites.
Regarding the steamed samples, we did not observe a sig-
nificant variation in bulk SAR values when compared to
Fig. 2. Acylation products (3) selectivities and PA conversions achieved over solid acids in the reaction between anisole and propanoic acid.
Please cite this article in press as: C. Bernardon, et al., Acidity versus metal-induced Lewis acidity in zeolites for FriedeleCrafts
acylation, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.03.008

C. Bernardon et al. / C. R. Chimie xxx (2016) 1e10
5
Fig. 3. SEM images of BEA (left) and ZSM-5 (right) crystals.
it takes place within the pores of H-BEA catalyst. In
contrast, FAU-type structure seems rather inefficient for
this reaction in terms of PA conversion.
obviously other factors must contribute to enhance the
activity. We tempt to state that acidity type and distribu-
tion are determinant factors for this reaction, in line with
former studies [7,30].
H-ZSM-5 and H-BEA zeolites are promising catalysts for
this acylation reaction, according to former studies
[24e27]. Their two peculiar pore topologies [28] seem
beneficial for achieving a high PA conversion and were
therefore selected as hosts for metallic sites. Fig. 3 presents
the SEM micrographs of BEA and MFI (parent-1) catalysts.
The BEA zeolite exhibits an arrangement of nanocrystals,
whilst ZSM-5 crystals possess a coffin-shape with few mi-
3.3. Influence of extra-framework aluminium content on the
activity
EFAl introduction into zeolites has already been proved
to enhance catalytic performances in several acid-catalysed
reactions [31]. The raise in activity was either attributed to
the creation of mesoporosity after steaming treatment [19]
or to a polarisation effect of cationic extra-framework
aluminium species present in the vicinity of Brønsted acid
sites [32].
crometers in size (3e5 mm). The pore accessibility may also
be a key parameter to reach a better catalytic performance
[29].
The influence of the SAR parameter in the present acid
catalysis has also been investigated (Table 1). The conver-
sion of propanoic acid remained roughly the same, 75% and
79% with SAR values of 11.5 and 12.5 for H-ZSM-5parent-2
and H-ZSM-5parent-1, respectively. Based on H/D exchange
technique (Table 1), H-BEAparent-1 exhibits a 45% higher
number of Brønsted sites with respect to H-BEAparent-2 and
yet the conversion reached higher values, i.e. 55% versus
25% (Table 2). This trend is usually observed since a higher
Al-content (low SAR) means ‘a priori’ more Brønsted sites
but also more Lewis sites.
EFAl species are often reported to be active acid sites,
even if their nature and how they affect the Brønsted
acidity of zeolites is still a matter of debate. EFAl species can
occupy cationic positions or be present as intra-zeolite
oxide species. It has been postulated that the latter spe-
cies may exist in different forms such as Al3þ, AlOþ,
þ
Al(OH)2 and AlOH2þ and also as monomeric AlOOH,
Al(OH)3 or even clusters [32]. Fig. 4 highlights the fact that
dealuminated zeolites with steam improved PA conversion
when compared to pristine zeolites, regardless of the
zeolite type. PA conversion was slightly increased from 43%
to 49% for BEA and from 62% to 69% for ZSM-5 samples
The BEA zeolite exhibits the highest density of Brønsted
sites but did not lead to the highest PA conversion (Fig. 2);
Table 2
Propanoic acid conversion and selectivity toward acylated products (ortho- and para) over Ag, Ni, and Fe loaded zeolites.
c
Catalysts
Si/Ala
PA Conversion %b
Selectivity 3 (ortho þ para)%b
Yield 3 (ortho þ para)%b
TOF (hꢀ1
)
H-ZSM-5parent-1
12.5
16.2
17.4
11.5
15.3
15.5
14.9
16.2
13.8
e
79
23
23
75
3
43
55
55
34
13
12
31
3
0.088
0.037
0.042
0.086
0.008
0.014
0.069
0.063
0.048
0.045
0.063
1% Ni-ZSM-5parent-1
4% Ni-ZSM-5parent-1
H-ZSM-5parent-2
2.6% Ag-ZSM-5parent-2
5.3% Ag-ZSM-5parent-2
H-BEAparent-1
1%Ni-BEAparent-1
H-BEAparent-2
0.25% Fe-BEAparent-2
10% Fe-BEAparent-2
41
100
100
70
73
76
4
4
55
26
25
20
13
39
19
19
15
13
77
100
14.8
a
SAR determined by XRF measurements.
Estimated from 1H NMR.
b
c
Calculations based on Brønsted acid site density (mmol 3/mmol Hþ/h).
Please cite this article in press as: C. Bernardon, et al., Acidity versus metal-induced Lewis acidity in zeolites for FriedeleCrafts
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6
C. Bernardon et al. / C. R. Chimie xxx (2016) 1e10
Fig. 4. Sequential extra-framework aluminium introduction and removal in BEA and ZSM-5 zeolites and their reactivity in the acylation reaction.
without a significant loss in selectivity toward acylated
products 3.
occurred during the acid-catalysed reaction. Such a detri-
mental effect in catalysis is most probably due to the
presence of silver as nanoparticles at the zeolite external
surface which may hinder the pore accessibility. The
presence of Ag nanoparticles in Ag-FAU was already evi-
denced by our team using TEM analysis (data not shown).
Indeed, for the 5.3% Ag-ZSM-5parent-2 sample, a symmetric
and narrow Ag 3d5/2 peak could be detected by XPS centred
at 368.6 eV. This value suggests that silver is exclusively
present in the zeolite as metallic Ag.
Furthermore, when EFAl species were removed using
EDTA as a complexing agent, PA conversions declined,
notably in the case of the ZSM-5 zeolite with smaller pore
size, even if the selectivity to acylation products 3 remained
nearly constant. These results showed that Lewis acidity
associated with EFAl species appears beneficial for the
catalytic process for both commercial catalysts.
3.4. Catalytic activity of metal-exchanged zeolites
Regarding nickel-based catalysts, an important decrease
in PA conversion was observed, from 79% to 23% with 1%
Ni-ZSM-5 and 4% Ni-ZSM-5 zeolites. The latter zeolites
have been thoroughly characterised using synchrotron
Hence, various transition metals, acting as Lewis acid
sites were exchanged within the zeolites to further try to
enhance the activity of the parent catalysts. All metal-
exchanged zeolites were evaluated as catalysts for the
acylation of anisole with PA and subsequently compared to
their pristine H-zeolite form.
techniques [18,33] and
a
mixture of cationic Ni2þ
compensation cation and NiO species formed within the
zeolite channels was ascertained. This is perfectly in line
with XPS data. The Ni 2p3/2 XPS spectra for 1% Ni-ZSM-
The catalytic performances of these metal-promoted ze-
olites at different loadings are listed in Table 2. These data
clearly assess a decrease in propanoic acid conversion over
all metal-doped catalysts compared with pristine zeolites.
However, thesemodifications inducedbythemetalpresence
are dependent on the nature and/or the metal loading.
5parent-1 and 4% Ni-ZSM-5parent-1 are presented in Fig. 5. For
the sample with 1% Ni loading, this peak could be decon-
voluted into three components, one centred at 854.6 eV
and the two others at 857.9 eV and 860.4 eV, respectively
(Fig. 5a). Taking into account that the binding energy (BE)
for unsupported NiO is 853.7 eV, the peak at 854.6 eV may
be assigned to Ni2þ in octahedral sites of the supported NiO
structure, and the peak at 857.9 eV may be associated with
Ni2þ in tetrahedral positions of a spinel-like structure of
NieAl oxide [34]. This tetrahedral environment might be
the result of the solid-state diffusion of Ni2þ into the pores
during the calcination process [35]. The last peak centred at
860.4 eV may be attributed to Ni2þ located inside the
zeolite channel as compensation cations.
In the case of the Ni 2p3/2 XPS spectrum of 4% Ni-ZSM-
5parent-1 only two contributions (855.6 eV and 857.8 eV)
could be detected, being assigned to the Ni cation after the
deconvolution procedure (Fig. 5b). Based on the discussion
regarding the Ni 2p3/2 spectrum of 1% Ni-ZSM-5parent-1, the
first peak centred at 855.6 eV corresponds to Ni2þ in NiO
species. The second peak agrees with a binding energy in
line with NieAl oxide structure. However, the first peak
3.4.1. Ag- and Ni-ZSM-5 catalysts
The XRF chemical analysis revealed that the Ag loading
reached 0.24 mmol/g after two cationic exchange steps.
Therefore, Ag ions have substituted approximatively one
sixth of the Brønsted acid site population in H-ZSM-5parent-2
(1.48 mmol/g). The incorporation of silver by an ionic ex-
change method into the H-ZSM-5parent-2 zeolite at a low
level of exchange (below 0.1%) might indicate the forma-
tion of isolated silver essentially located as a compensation
cation in the zeolite framework. However,
a drastic
decrease in the PA conversion, compared with pristine H-
ZSM-5 zeolite, was noticed from 75% to 3e4% after doping
with silver. Despite the low silver loading in the zeolite
(0.01 wt% after one cationic exchange and 0.03 wt% after
the second exchange), it appears that a strong inhibition
Please cite this article in press as: C. Bernardon, et al., Acidity versus metal-induced Lewis acidity in zeolites for FriedeleCrafts
acylation, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.03.008

C. Bernardon et al. / C. R. Chimie xxx (2016) 1e10
7
Satellites
a)
870
865
860
855
850
Binding Energy (eV)
Satellites
b)
870
865
860
855
850
Binding Energy (eV)
Fig. 5. XPS spectrum of Ni 2p3/2 for 1% Ni-ZSM-5parent-1 (a) and 4% Ni-ZSM-5parent-1 (b).
shifts to higher BE values from 854.6 to 855.6 eV at higher
nickel loading. This new BE value suggests that the envi-
ronment of Ni2þ ions in Ni (4% wt) is different from the one
in Ni (1% wt) and may be assigned to Ni2þ in Ni(OH)2 as
proposed by Grosvenor et al. [36]. Furthermore, it is note-
worthy that 4% Ni-ZSM-5parent-1 exhibits a more obvious
first peak than in the zeolite with lower Ni loading, thus
indicating a larger amount of Ni2þ on the surface than in
the 1% Ni-ZSM-5parent-1 sample. This increase in the in-
tensity of the low energy peak can be the consequence of
the decreased diffusion of Ni2þ in the zeolite pores.
species. In terms of selectivity, a loading with more nickel
species did not alter the formation of acylated products 3.
Whatever the nature or the amount of the metal used (Ni,
Ag), pristine H-ZSM-5 zeolites proved to be the most active
and selective catalysts with yields in the range of 30e40%
toward the desired products (Table 2).
3.4.2. Ni- and Fe-BEA zeolites
The acylation of anisole was also performed over nickel
and iron-doped BEA zeolites. Likewise to Ni-ZSM-5 zeo-
lites, 1 wt% Ni-BEAparent-1 led to a large decrease in PA
conversion from 55% to 26% over BEAparent-1 (Table 2).
Surprisingly, the same selectivity in acylation products was
The detrimental effect of nickel species on the Frie-
deleCrafts acid catalysis is less pronounced than for silver
Please cite this article in press as: C. Bernardon, et al., Acidity versus metal-induced Lewis acidity in zeolites for FriedeleCrafts
acylation, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.03.008

8
C. Bernardon et al. / C. R. Chimie xxx (2016) 1e10
reached and TOF values were equal (this parameter is
formulated as the quantity of acylated products formed (3)
per mole of acid sites in one hour); this behaviour is
different from the one observed for Ni-ZSM-5 zeolites. The
loading of 1 wt% of Ni by impregnation on BEA zeolites may
favour the formation of small nickel particles in contrast to
its distribution as compensation cation. As a result, the
lower PA conversion reached over Ni-BEA zeolites could be
ascribed to faster coke formation on nickel or NiO particles.
It is also surprising that for ZSM-5parent-1 and BEAparent-1
zeolites promoted with the same Ni content (1 wt%)
exhibited similar PA conversion (23% and 26%, respec-
tively). However, at a similar Ni loading (1 wt%), the
decrease in the conversion was more pronounced for the
ZSM-5 zeolite than that for the BEA zeolite compared to
their related pristine materials. This behaviour may be
attributed to different natures of Ni species or different
distributions depending on the zeolite structure as well as
the pore system in BEA zeolites. A higher anisole and car-
boxylic acid accessibility should be allowed in the BEA
zeolite framework. Despite the decrease in PA conversions
upon Ni introduction into BEA zeolites, no significant
variation in ortho- and para-acylation products (3) could be
observed.
0.25 wt% and 10 wt% Fe-BEAparent-2 zeolites also
exhibited lower PA conversions (20% and 13%, respectively)
when the amount of iron was enhanced with respect to the
parent BEA zeolite. The significant decrease in SSA values
observed between H-BEAparent-2 (485 m2/g) and 10 wt% Fe-
BEAparent-2 zeolites (379 m2/g) in Table 1 suggests the
presence of Fe oxides probably as disordered crystallites
due to the absence of characteristic peaks in the XRD
pattern [37]. Concerning the sample 10% Fe-BEAparent-2, the
Fe 2p3/2 XPS spectra could be described by two compo-
nents, one centered at 710.6 eV and the second at 711.8 eV.
According to Gurgul et al. [38], all BE of the Fe3/2 lines are
higher than 710.5 eV, which strongly suggests that iron is
present as Fe3þ species in two different environments.
zeolites. It is therefore questionable whether such
hampered catalytic behaviour is rather related to the cre-
ation of metallic Lewis acid sites or to the decrease of
Brønsted acid sites or to their limited accessibility.
3.5. Rationalization of the active acid site nature
The respective role of Brønsted acid sites and metal
species in the catalytic behaviour of zeolites is still
controversial to date. Consequently, we were interested in
the identification of the nature of the acid sites really acting
as active species to get insight into the understanding of
how metal-modified zeolites behave as catalysts. Generally,
it is claimed that the reactants could be activated on
Brønsted and/or Lewis acid sites present in metal-loaded
zeolites [17]. Two activation paths may therefore be ex-
pected: i) either direct involvement of the metal species
and/or ii) direct involvement of the remaining Brønsted
acid sites as active sites. According to the data presented in
Table 2, PA conversion was not directly proportional to the
density of Brønsted acid sites, this feature might demon-
strate that it is not possible to consider the sole Brønsted
acid sites for performing the FC reaction.
The density and nature of the two kinds of acid sites
were carefully considered to tempt any rationalization of
the catalyst behaviour. It has already been assessed that the
accessibility and the relative content in Lewis and Brønsted
acid sites strongly depend on several factors, namely the
topology of the zeolite, the SAR, and the water formation
during the reaction under batch conditions at 423 K which
may complicate the discrimination between Lewis and
Brønsted sites [17]. As reported elsewhere [22], the cata-
lytic performances might be modulated by a possible
involvement of species generated after calcination such as
additional dispersed metal oxide species. Hence, the in-
fluence of these species on the catalysis cannot be
completely ruled out. The investigation of the role of
metallic species on catalyst's activity can be complicated by
the difficulty to identify their structure.
€
Furthermore, the same authors excluded by Mossbauer
spectroscopy the presence of non-framework iron (Fe3O4
and Fe2O3).
Regarding the aforementioned observations, metal-
modified zeolites were less effective in the FriedeleCrafts
reaction than their parent counterparts, regardless of the
nature of the metal species. Bare protonic zeolites exhibited
higher activity than Lewis acid metal-zeolites which high-
light the detrimental role of metal species incorporated in
the zeolites (even limited in the case of iron). Indeed, the PA
conversion diminished according to the following order: H-
zeolite > Fe > Ni > Ag. The influence of Ag on PA conversion
was more significant compared with the other metals, due
to higher Lewis acidity of Agþ exchanged over zeolites. In
addition, even after incorporation at higher amounts into
BEA zeolites (ꢂ7 wt%), the presence of iron led only to a
moderate decrease in PA conversion with respect to H-BEA
zeolites. It is important to point out that the classification is
correlated to the strength of the Lewis acid sites created by
exchanged metal cations. The higher the Lewis acidity
(Agþ > Ni2þ > Fe3þ), the more severe decrease in activity
could be observed.
This comparison showed that the catalyst without any
iron introduction (BEA zeoliteparent-2) is still more active
than Fe-doped BEA zeolites, generating a slightly higher
yield in ortho- and para-acylation products (3) (i.e. 19% for
H-BEAparent-2 versus 15% and 13% for Fe-BEA zeoliteparent-2).
It is important to point out that the negative effect of metal
introduction is less pronounced for Fe-doped zeolites.
Indeed, TOF values are comparable with pristine H-zeolites,
indicating that the intrinsic catalyst performance is equal.
In addition to their negative role related to the substitution
of protons, silver and nickel species also appear detrimental
in terms of TOF, suggesting an inhibiting effect of metallic
species. In contrast, iron species appear rather inactive for
this FC acid-catalysed reaction. The same tendencies were
observed by Cruz-Cabeza et al. [23] who highlighted a non-
beneficial effect of Ni in BEA zeolites on acylation reactions.
At this point of the study, it is worth mentioning that
more or less drastic decreases in activity and selectivity
were obtained over metal-loaded zeolites. This can be
assigned to the modification of the acidity in the host
In contrast to what is usually considered for other re-
actions [33,39,40], the relative enrichment of zeolites in
Lewis acidity seems detrimental for PA conversion. The
Please cite this article in press as: C. Bernardon, et al., Acidity versus metal-induced Lewis acidity in zeolites for FriedeleCrafts
acylation, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.03.008

C. Bernardon et al. / C. R. Chimie xxx (2016) 1e10
9
reactivity of all zeolites was negatively impacted by the
introduction of Lewis acidity induced by the metals. Mnþ
might stabilize too strongly the intermediate species than
remaining Hþ and therefore rather act as a limiting actor
toward reactivity. Moreover, there might be an adsorption
competition between the reactants on Lewis and Brønsted
acid sites, leading to slower acylation rates. The lower
selectivity exhibited by promoted zeolites can be attributed
to the presence of strong Lewis acid sites that are able to
catalyse consecutive reactions, leading to the formation of
undesired products. This confirms that metal species are
not necessary to yield acylated products 3.
The content and accessibility of metals (present as ox-
ides or metal cations) play a role in the modification of
acidic properties by modulating the density of Brønsted
acid sites. The addition of metals may not only affect the
strength of these acid sites but also diminish their con-
centration. Indeed, ion exchange and impregnation
methods resulted in a decrease of Brønsted acidity while
new Lewis acid sites were generated (Table 1). The TOF was
calculated, assuming that all Brønsted groups can act as
catalytically active sites, to reveal the intrinsic activity of
the samples. For the Ni-ZSM-5parent-1 samples, TOF values
remained nearly identical for H-ZSM5parent-1 (0.088 mmol
3/mmol Hþ/h). TOF diminished over 1 wt% Ni-ZSM5parent-1
(0.037 mmol 3/mmol Hþ/h) and 4 wt% Ni-ZSM5parent-1
(0.042 mmol 3/mmol Hþ/h). This may be associated with
the presence of inactive metallic species formed at a
loading of up to 1 wt%.
product desorption. In contrast, the presence of metals in
the ZSM-5 zeolite led to seriously diminish TOF values. In
addition to substitute active protons for the reaction, metal
species further prevent the smooth progress of the reac-
tion. This might probably be due to a hindered accessibility
in ZSM-5 medium-sized pores. The dependence of TOF
values with Brønsted acidity on the ZSM-5 surface supports
the higher probability for the FC reaction to proceed at the
external surface (or at the pore-mouth entrance) as high-
lighted previously (based on data presented in Fig. 2).
However, a general tendency clearly shows that TOF
values achieved over doped zeolites are globally lower than
those of pristine zeolites, disclosing the incorporation of
metal ions as a crucial parameter to reach a high activity in
the FriedeleCrafts reaction. This consideration strongly
supports that the active acid site is of Brønsted nature, as
reported for
[7,29,41,42].
a large palette of catalytic reactions
One could consider that metal sites are competing with
Brønsted sites as active sites involved in the catalytic pro-
cess but the more relevant contribution is certainly attrib-
uted to Brønsted acid sites. Consistently, no acylation
product was obtained when the Na-zeolites were
employed as catalysts, reinforcing the direct involvement of
Brønsted acid sites in the formation of acylation products. It
has already been reported that the higher density of acid
sites is a crucial parameter to enhance the activity in the
acylation reaction [43]. The presence of numerous acyl
carbocations, generated by a higher density of Brønsted
acid sites, impacts the electrophilic substitution with the
carboxylic acid to produce acylated products.
Many reactions catalysed by Brønsted acidity have been
studied in the past. In the present study, the importance of
this type of acid sites was checked again. A direct com-
parison between H-zeolites and Ag, Ni, and Fe was made
and revealed the prominence of Brønsted acid over Lewis
acid solids. Our experimental results emphasize that the
Surprisingly, two distinct relationships between TOF
values and the density of Brønsted acid sites could be
established for BEA and ZSM-5 zeolites (Fig. 6). Whilst the
TOF only suffered minor modifications as a function of the
density of Brønsted acid sites in BEA, the ZSM-5 zeolite
exhibited a continuous TOF raise with increasing acidity. It
indicates that the presence of metallic species in BEA
channels neither hinder the reactant adsorption nor the
Fig. 6. TOF value dependence to the Brønsted acid site density (BEA and ZSM-5 zeolites).
Please cite this article in press as: C. Bernardon, et al., Acidity versus metal-induced Lewis acidity in zeolites for FriedeleCrafts
acylation, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.03.008

10
C. Bernardon et al. / C. R. Chimie xxx (2016) 1e10
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Please cite this article in press as: C. Bernardon, et al., Acidity versus metal-induced Lewis acidity in zeolites for FriedeleCrafts
acylation, Comptes Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2016.03.008