Identification of active surface species for friedel-crafts acylation and koch carbonylation reactions by insitu solid-state NMR spectroscopy

Article abstract of DOI:10.1002/anie.201209907

Finding the culprits: Insitu NMR spectroscopy combined with theoretical calculations show the formation of acetyl species covalently bound to framework oxygen atoms in acid zeolites. These species, and not the usually assumed acylium cations, are the reac

Full text of DOI:10.1002/anie.201209907

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DOI: 10.1002/anie.201209907
Reaction mechanisms
Identification of Active Surface Species for Friedel–Crafts Acylation
and Koch Carbonylation Reactions by in situ Solid-State NMR
Spectroscopy**
Inꢀs Lezcano-Gonzꢁlez, Josꢀ A. Vidal-Moya, Mercedes Boronat, Teresa Blasco,* and
Avelino Corma
The acid-catalyzed Friedel–Crafts acylation reaction of aro-
matic rings with acylating agents yields aromatic ketones
(Scheme 1) which are common intermediates for the produc-
zeolites have already found industrial application as Frie-
del–Crafts acylation reaction catalysts,[3] although their uni-
versal use is still limited by low activity and selectivity, and by
catalyst deactivation.
Scheme 1 illustrates the general mechanism of Friedel–
Crafts acylation on solid acids for the reaction of anisole with
acetyl chloride. It is usually assumed that the acylating agent
reacts with the Brønsted acid site to form an acylium cation
+
À
=
(H3C C O), which then attacks the aromatic ring to give
electrophilic substitution.[4] In situ magic angle spinning
(MAS) NMR spectroscopy has shown that acetyl chloride
adsorbed on the AlCl3 Lewis acid catalyst gives rise to the
+
À
=
formation of H3C C O, which is characterized by two
signals at d 13C = 152 ppm and d 13C = 14 ppm for the carbonyl
and methyl groups, respectively.[5] On the other hand, the 13
C
MAS NMR spectra of acetyl chloride adsorbed on a series of
zeolites show a signal at d 13C (CO) = 180 ppm, which is
attributed to acetyl species covalently bound to a zeolite
framework oxygen atom (Z-CO-CH3),[6] whereas a second
signal at d 13C (CO) ca. 160 ppm was tentatively assigned to
+
À
=
free H3C C O species. However, the interpretation of the
13C NMR spectra was not definitive and the issue of which
was the reactive species was unclear.[6]
Aside from Friedel–Crafts acylation, other reactions on
solid acid catalysts, such as the Koch carbonylation of alcohols
or olefins with CO, are assumed to occur through acylium or
acetyl species intermediates.[7–18] Scheme 2 shows the mech-
anism of methanol carbonylation on solid acids. Acylium/
acetyl species are formed by insertion of CO into the methoxy
Scheme 1. Friedel–Crafts acylation of anisole over acid zeolites.
tion of fragrances, pesticides, and drugs.[1,2] The industrial
process uses Lewis acids as catalysts, typically AlCl3 and other
metal halides, and is associated with the production of a large
amount of waste. Thus, the development of new processes
that satisfy global environmental regulations is imperative.
A variety of solids, such as micro- and mesoporous
aluminosilicates, oxides, clays, and heteropolyacids, have
been tested as catalysts in the Friedel–Crafts acylation of
aromatic substrates with acyl chlorides, anhydrides, and
carboxylic acids as acylating agents.[1,2] Among them, large-
pore zeolites are the most promising candidates, because of
the large variety of structures available and the possibility of
tailoring the acidity, hydrophobic/hydrophilic properties, and
selectivity for the para-substituted products.[1,2] Indeed,
[*] Dr. I. Lezcano-Gonzꢀlez, Dr. J. A. Vidal-Moya, Dr. M. Boronat,
Dr. T. Blasco, Prof. A. Corma
Instituto de Tecnologꢁa Quꢁmica (UPV-CSIC), Universidad
Politꢂcnica de Valencia—Consejo Superior de Investigaciones
Cientꢁficas, Av. Naranjos s/n, 46022 Valencia (Spain)
E-mail: tblasco@itq.upv.es
[**] The authors acknowledge Spanish MINECO (Projects MAT-2012-
38567-C02-01, CTQ-2012-37925-C03-01 and Consolider Ingenio
2010-MULTICAT, CSD2009-00050).
Scheme 2. Carbonylation of methanol with CO over acid zeolites.
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(or, more generally, alkoxy) species generated by the reaction
of an alkene or alcohol molecule with a Brønsted acid site of
+
À
=
À
the catalyst. The H3C C O or -CO CH3 species can be
subsequently captured by water to produce acetic acid (Koch
reaction), by methanol or dimethyl ether to give methyl
acetate, or by ammonia to yield acetamide.[7–18] An acetyl
species bound to an oxygen atom of the catalyst has been
reported to be the intermediate in the carbonylation of
methanol on 12-tungstophosphoric acid catalyst (12-
H3PW12O40), based on the appearance of 13C MAS NMR
signals at d 13C (CO) = 193 ppm, and d 13C (H3C-) = 22 ppm
attributed to CH3-CO-(O-W-) species.[16] Similar surface
acetyl groups, Z-CO-CH3, have been hypothesized to be
possible intermediates in the Koch carbonylation of methanol
and dimethyl ether over H-mordenite,[10,11,13,18] and of meth-
ane over Zn-ZSM-5 catalysts.[19] However, other authors
assume that, on acid zeolites, these reactions occur through
+
À
=
the formation of H3C C O species as unstable intermedi-
ates too reactive to be detected by NMR spectroscopy.[7–9]
The aim of this work is to elucidate whether covalent H3C-
+
À
=
CO-Z or ionic H3C C O species are formed in zeolites by
reaction of an acylating agent with a Brønsted acid site, and
by carbonylation of an alkoxy group. For this purpose, we
have combined theoretical calculations and in situ solid-state
NMR spectroscopy to study the acylation of anisole over
acidic H-Beta (Scheme 1) and the carbonylation of methanol
over acidic H-Mordenite zeolite (Scheme 2). The results
obtained provide direct experimental evidence of the for-
mation acetyl species covalently bound to framework oxygen
as the reactive intermediates in Friedel–Crafts acylation and
Koch carbonylation reactions on acidic zeolites.
Figure 1. Calculated 13C isotropic chemical shifts of a) acetyl chloride
adsorbed on a Brønsted acid site, b) acetyl group covalently bound to
a zeolite framework oxygen, c) cationic acylium–zeolite complex,
d) para-methoxy acetophenone adsorbed on a Brønsted acid site,
e) acetic acid adsorbed on a Brønsted acid site, f) covalently bound
acetyl interacting with acetic acid, and g) covalently bound acetyl
interacting with HCl. The numbers in italics are the calculated
isotropic 13C NMR chemical shifts of the associated carbon atoms.
Figure 1 shows the calculated 13C NMR isotropic chemical
shifts (13C d) of the methyl and carbonyl groups of acetyl
chloride adsorbed through hydrogen bonds onto the zeolite
Brønsted acid site (Figure 1a), and of the two possible species
resulting from the subsequent reaction and release of hydro-
gen chloride: the covalent acetyl–zeolite (Figure 1b) and the
cationic acylium–zeolite (Figure 1c) species. The 13C NMR
chemical shifts calculated for the acylium–zeolite complex
shown in Figure 1c agree fairly well with experimental values
+
À
=
for this H3C C O species on AlCl3, (carbonyl and methyl
groups at d = 152 ppm and d = 14 ppm, respectively), which
supports the validity of the theoretical models. Figure 1d,e
shows the 13C NMR chemical shifts of para-methoxy aceto-
phenone (PMAP) obtained by acylation of anisole, and of
acetic acid produced by the carbonylation of methanol with
CO, respectively, interacting with a zeolite Brønsted acid site.
According to the theoretical chemical shifts shown in
Figure 1, it should be possible to distinguish between the
carbonyl groups of the two reaction intermediates (acetyl–
zeolite at d = 169.7 ppm and acylium–zeolite at d = 154.6), as
they are separated by more than 15 ppm and do not overlap
with the reactants and products involved in the two reactions.
As the 13C NMR chemical shifts of the methyl group are less
conclusive, for the sake of clarity we shall mostly focus on the
carbonyl region of the 13C NMR spectra.
Figure 2. 13C MAS NMR spectra of the 13CH313COCl/anisole (molar
ratio=1:1) reaction mixture a) adsorbed at room temperature on H-
Beta zeolite. The same sample was then heated at 363 K for b) 90 min
and c) 300 min. Asterisks denote spinning side bands.
temperature on H-Beta zeolite (Figure 2a) and after subse-
quent heating at 363 K for different periods of time (Fig-
ure 2b,c). The spectrum of Figure 2a, shows two carbonyl
bands at d = 184 ppm and at d = 168 ppm. According to
theoretical calculations, the band at 184 ppm with a methyl
Figure 2 shows the carbonyl region of the 13C MAS NMR
spectra of the reaction mixture for the Friedel–Crafts
acylation, 13CH313COCl/anisole (1:1), adsorbed at room
signal at
d
13C = 32 ppm (not shown) is assigned to
13CH313COCl adsorbed on a zeolite Brønsted acid sites
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(Figure 1a), and the band at 168 ppm must correspond to
a covalently bound acetyl–zeolite group (Figure 1b). Heating
the sample at 363 K for 90 min provokes the appearance of
a new signal at d = 208 ppm, which corresponds to para-
methoxy(13CO)-acetophenone (PMAP) adsorbed on the
zeolite (Figure 1d). We must note that adsorbed PMAP
appears about 10 ppm high-field shifted with respect to the
free compound (d = 198 ppm); this is indicative of strong
adsorption on the zeolite, which can contribute to catalyst
deactivation.[20] The spectrum of Figure 2c shows that, at
longer reaction times, the intensity of the signal of PMAP at
208 ppm progressively increases at the expense of the signal at
168 ppm, which indicates that the covalently bound acetyl–
zeolite species is a reactive intermediate in the Friedel–Crafts
acylation reaction. It is worth noting the absence of any signal
ite zeolite. The carbonyl region of the 13C NMR spectrum of
the reaction mixture heated at 473 K on H-Mordenite (Fig-
ure 3c) exhibits a band at 183 ppm that can be attributed to
1-2-13C acetic acid or methyl acetate, and another one at
170 ppm that corresponds to the covalent Z-13CO-13CH3
complex, which disappears at longer reaction times. A
comparison of the spectra shown in Figure 3 makes evident
the similarity of the intermediate species involved in the two
reactions, Friedel–Crafts acylation and carbonylation, on acid
zeolites.
In summary, in situ solid-state NMR spectroscopy com-
bined with DFT calculations has demonstrated the formation
of a covalent acetyl–zeolite complex as reactive intermediate
species in both Friedel–Crafts acylation and Koch type
carbonylation, while no signal that could be attributed to
acylium cation, the usually assumed intermediate species, has
been observed in any spectra.
À
at 150–160 ppm in Figure 2 that could be attributed to H3C
+
=
C
O (Figure 1c), which suggests that this species is not
formed.
The signal at d 13C = 32 ppm (not shown), which is
characteristic of the methyl groups of acetyl chloride
(Figure 1), disappears in the spectra of Figure 2b,c; thus
indicating that this compound has reacted, and that the peak
at d 13C = 180 ppm must correspond to other species. Accord-
ing to the theoretical chemical shifts reported in Figure 1, this
is most probably due to acetic acid (Figure 1e) produced by
partial hydrolysis of acetyl 1-2-13C chloride (Figure 1a) or of
acetyl 1-2-13C-zeolite (Figure 1b) with residual water present
in the reaction media. However, hydrogen bonded species
such as those depicted in Figure 1 f,g, in which HCl or acetic
acid are interacting with the carbonyl group of the acetyl–
zeolite species, could also contribute to the signal at d =
184 ppm. Indeed, this would explain the increase in the
intensity of the signal at 168 ppm of acetyl–zeolite in the
spectrum of Figure 3b, recorded after degassing at 333 K, as
compared to the spectrum of Figure 3a, which was recorded
without degassing.
Experimental Section
Acylation reaction: Before any adsorption, zeolite H-Beta (CP811,
Zeolyst International) was introduced into
a glass insert and
dehydrated in a vacuum line at 673 K overnight to a final pressure
of 10À5 mbar. Then, 13CH313COCl(1000 mmol/g zeolite; 13C, 99%;
provided by Isotec) corresponding to a molar ratio of 13CH313COCl/
Al = 0.5, was introduced onto the activated zeolite before heating the
sample at 333 K for 2 h without degassing. The glass insert containing
the sample was sealed and used for recording NMR spectra. A second
sample was prepared in a similar way, but before being sealed it was
heated for 1 h at 333 K under vacuum. To study the acylation reaction,
a third sample was prepared in a similar way as the latter, but with
a molar ratio of 3CH313COCl/anisole = 1 added before sealing the
glass insert. To follow the reaction, the sample was treated at 363 K
outside of the NMR probe, progressively increasing the reaction time.
In all cases, sealing was carried out with a torch while the glass insert
was immersed in liquid nitrogen.
Koch reaction: H-Mordenite was obtained by calcination of
a
commercial mordenite (ammonium form) with a Si/Al = 10
To determine whether covalently bound acetyl–zeolite is
also the intermediate species in carbonylation reactions, we
studied the reaction of 13CH3OH with 13CO over H-Morden-
(CBV20A, Zeolyst International) at 673 K over 2 h. Subsequently,
13CH3OH (730 mmol/g zeolite; 13C, 99%; Cambridge Isotope Labo-
ratories) and then 13CO (2200 mmol/g zeolite; 13C, 99%; Isotec) were
admitted into the glass insert immersed into liquid nitrogen. The
molar ratios used were 13CH3OH/Al = 1:2 and 13CO/13CH3OH =
ca. 3:1. The glass inserts were sealed while they were immersed in
liquid nitrogen. The NMR spectra were recorded after the sample was
treated at 473 K outside of the NMR probe with progressively
increasing reaction time.
Solid-state NMR spectra were recorded at room temperature on
a Bruker AV 400 WB spectrometer. 13C MAS NMR spectra were
recorded with proton decoupling, a 908 pulse of 5 ms, and a recycle
delay of 15 s. The 1H to 13C (1H/13C) CP-MAS spectra were recorded
with a 908 pulse for 1H of 5 ms, a contact time of 3.5 ms, and a recycle
delay of 5 s.
Computational details: The Brønsted acid site was simulated by
means of a Al(OSiH3)3(OH)SiH3 cluster of atoms that was cut out
from the periodic structure of a BEA zeolite, as described in previous
work.[21] It consists of one Al atom, the four OSi groups in the first
coordination sphere, the proton of the Brønsted acid site, and the
H atoms that were used to saturate the dangling bonds that connected
the cluster to the rest of the solid. The geometries of the acid site,
isolated molecules, and complexes resulting from adsorption of
reactants and products on the acid site were optimized using density
functional theory at the B3PW91 level[22,23] with the standard
6-31G(d,p) basis set,[24] as implemented in the Gaussian 03 computer
Figure 3. 13C CP-MAS NMR spectra of 13CH313COCl adsorbed on H-
Beta zeolite at room temperature and a) heated at 333 K for 2 h,
before being b) outgassed at 333 K for 1 h. c) The reaction mixture
13CO/13CH3OH (molar ratio=3:1) on H-Mordenite zeolite heated at
473 K for 1 min. Asterisks denote spinning side bands.
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program.[25] In these calculations, the coordinates of all atoms were
fully optimized, except for the terminal H atoms of the SiH3 groups,
which were kept fixed at their original positions to avoid unrealistic
deformations. Isotropic absolute chemical shielding constants (s)
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shifts were calculated as d = srefÀs, and corrected with equations
obtained from a preliminary study of the performance of the B3PW91
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Keywords: acid catalysts · density functional theory ·
heterogeneous catalysis · NMR spectroscopy · zeolites
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