Surface species formed during aniline methylation on zeolite H-Y investigated by in situ MAS NMR spectroscopy

Article abstract of DOI:10.1006/jcat.2001.3300

Aniline alkylation with methanol on zeolite H-Y has been studied using in situ ¹³C MAS NMR spectroscopy under batch conditions. To clarify the main reaction pathways, the conversion of methanol as well as the interaction of aniline with surface methoxy groups were investigated under similar conditions. Methanol-¹³C and methyl iodide-¹³C were used as labeled reactants. Co -adsorption of aniline and methanol-¹³C on zeolite H-Y led to strongly adsorbed aniline molecules, assigned to aniline H-bonded to zeolite Bronsted acid sites, and three types of methanol species of different mobility: mobile methanol molecules with a liquid-like characteristics and two types of rigid methanol species with solid-like characteristics attributed to a methanol adsorption complex with aniline and surface methoxy groups, respectively. Among all the methanol species observed, only surface methoxy groups were shown to be responsible for aniline alkylation which takes place at temperatures from 373 to 523 K. The formation of surface methoxy groups was found to be a limiting step of the overall reaction. The primary alkylation product is N-methylaniline. Toluidines and N-methyltoluidines are formed at temperatures from 523 to 623 K after complete conversion of methanol to N-methylaniline. Therefore, isomerization or disproportionation of N-methylaniline was proposed to account for their formation.

Full text of DOI:10.1006/jcat.2001.3300

Journal of Catalysis 203, 375–381 (2001)
doi:10.1006/jcat.2001.3300, available online at http://www.idealibrary.com on
Surface Species Formed during Aniline Methylation on Zeolite H–Y
Investigated by in Situ MAS NMR Spectroscopy
Irina I. Ivanova, ,1 Elena B. Pomakhina, Alexander I. Rebrov,† Michael Hunger,‡
Yuryi G. Kolyagin, and Jens Weitkamp‡
Department of Chemistry, Moscow State University, Leninskie Gory, 119899 Moscow, Russia; †Institute of Petrochemical Synthesis, RAS,
Leninsky pr. 29, 117012 Moscow, Russia; and ‡Institute of Chemical Technology, University of Stuttgart, 70550 Stuttgart, Germany
Received February 23, 2001; revised May 4, 2001; accepted June 1, 2001
mers leading to catalyst deactivation (6–8). It is known that
alkylation of aniline may take place on acidic (3–6) and
on basic zeolites (7, 8). Moreover, this reaction was per-
formed even on zeolites with redox properties (3, 9, 10).
A detailed understanding of the mechanisms of the main
and side reactions occurring during alkylation on various
types of zeolites and the determination of the active sites
and their role in each elementary step may give a key to the
design of new solid catalysts with the required properties.
Such mechanistic information can be obtained by recently
developed in situ MAS NMR spectroscopic technique
(11–19). Among these studies, the investigations on the
interaction of methanol with ammonia over mordenite (20)
and H–RHO and H–SAPO-34 (21) catalysts are the clos-
est to the above subject. However, according to our know-
ledge, no direct data are available as yet on the mechanism
of aniline methylation on zeolites.
Anilinealkylation with methanolon zeoliteH–Yhas been studied
using in situ 13C MAS NMR spectroscopy under batch conditions.
To clarify the main reaction pathways, the conversion of methanol
as well as the interaction of aniline with surface methoxy groups
were investigated under similar conditions. Methanol-13C and
methyl iodide-13C were used as labeled reactants. Co -adsorption of
aniline and methanol-13C on zeolite H–Y led to strongly adsorbed
aniline molecules, assigned to aniline H-bonded to zeolite Brønsted
acid sites, and three types of methanol species of different mobility:
mobile methanol molecules with a liquid-like characteristics and
two types of rigid methanol species with solid-like characteristics
attributed to a methanol adsorption complex with aniline and sur-
face methoxy groups, respectively. Among all the methanol species
observed, only surface methoxy groups were shown to be responsi-
ble for aniline alkylation which takes place at temperatures from
373 to 523 K. The formation of surface methoxy groups was found
to be a limiting step of the overall reaction. The primary alkylation
product is N-methylaniline. Toluidines and N-methyltoluidines are
formed at temperatures from 523 to 623 K after complete conver-
sion of methanol to N-methylaniline. Therefore, isomerization or
disproportionation of N-methylaniline was proposed to account for
c
This study aims at a clarification of the mechanism
of aniline methylation over acidic zeolite Y. Controlled-
atmosphere 13C MAS NMR spectroscopy under batch con-
ditions using samples sealed in glass cells was employed for
this purpose. The attention is focused on the following:
their formation.
2001 Academic Press
1. identification of the reaction intermediates which may
play a role as alkylating agents;
INTRODUCTION
2. determination of the primary and secondary reaction
products;
3. investigation of the mechanisms leading to C- and
N-alkylation.
Methylation of aniline is an industrially important pro-
cess, aimed at the synthesis of mono-N-methylaniline,
di-N-methylaniline, and toluidine, which are useful raw
materials for organic syntheses as well as important inter-
mediates in the dye-stuff production and in the pharma-
ceutical and agrochemical industries. Until now, industrial
processes leading to these products are based on the appli-
cation of corrosive liquid acids as catalysts (1, 2) and should
be replaced by environmentally more benign processes us-
ing solid catalysts such as oxides, clays, and zeolites (3–8).
However, the formation of the target alkylation products
on these catalysts is often accompanied by the formation of
by-products such as various C-alkylated anilines and poly-
EXPERIMENTAL
Materials
Zeolite Na–Y with an nSi/nAl ratio of 2.6 was obtained
from Union Carbide (Tarrytown, NY). The H+ form of the
zeolite (H–Y) was prepared by fourfold ion exchange in an
1 M aqueous solution of NH4NO3 and subsequent cal-
cination at 673 K (vide infra). The material was charac-
terized using XRD, electron microscopy, atomic emission
spectroscopy with inductively coupled plasma (AES-ICP),
1 To whom correspondence should be addressed.
375
0021-9517/01 $35.00
c
Copyright
2001 by Academic Press
All rights of reproduction in any form reserved.

376
IVANOVA ET AL.
1
FTIR, solid state H, 27Al and 29Si MAS NMR spectro- polarization (contact time 3 ms) were performed to distin-
scopy, nitrogen adsorption, and temperature-programmed guish between species of different mobilities. Repetition
desorption of ammonia. 27Al and 29Si MAS NMR indicated times of 5 s were used both for the MAS and CP/MAS
that no dealumination occurred and that the Si/Al ratio NMR measurements. For each spectrum, between 400 and
in the zeolite framework is close to those determined by 8000 free induction decays were accumulated with a sample
chemical analysis. 1H MAS NMR spectrum corresponded spinning rate of 3.0–3.5 kHz.
to the spectrum of an intact H–Y zeolite (18).
In a typical NMR experiment, the sealed sample cell was
The labeled reactants methanol-13C (99.9% enriched) rapidly heated to the selected temperature outside the spec-
and methyl iodide-13C (99.9% enriched) were purchased trometer and maintained at this temperature for a given
from ICON Services Inc. and Sigma–Aldrich, respectively. period of time. MAS NMR spectra were recorded at 293 K
Aniline was obtained from Promochem.
after the sample cell was quenched in liquid nitrogen. After
the NMR data were collected, the sample cell was returned
to reaction conditions. The experimental temperature was
increased stepwise, and spectra were recorded as a function
of temperature.
Controlled-Atmosphere 13C MAS NMR Measurements
Controlled-atmosphere in situ NMR experiments were
performed in sealed glass cells containing the loaded cata-
lyst and an adsorbate and fitting precisely into 7-mm
Bruker MAS NMR rotors. The powdered catalyst samples
(0.08 0.01 g) were packed into the NMR tubes (Wilmad,
5.6-mm o.d. with constrictions), evacuated to a final pres-
RESULTS AND DISCUSSION
3
sure of 10 Pa after heating for 12 h at 673 K, and cooled
1. Adsorption of the Reactants Methanol and Aniline
down to 298 K before the adsorption of reactants. In dif-
ferent experiments, the catalyst samples were loaded with
methanol-13C, aniline, or mixtures of aniline and methanol-
13C (Table 1). For some experiments, the zeolite samples
were methoxylated prior to aniline adsorption by treat-
ment with 13C-labeled CH3OH or CH3I at 433 K for 5 h.
The treated samples were then evacuated at 393 K to re-
move unreacted methoxylating agents. After adsorption of
all reactants, the NMR cells, maintained at 77 K to ensure
a quantitative adsorption, were carefully sealed to achieve
proper rotor balance and high spinning rates in the MAS
NMR probe.
Figure 1 shows in situ 13C MAS NMR and 13C CP/MAS
NMR spectra observed after adsorption of aniline, me-
thanol-13C, and an aniline–methanol-13C mixture on zeo-
lite H–Y. Adsorption of aniline leads to a broad sideband
pattern with 13C MAS NMR central lines at ca. 122 and
129 ppm. The occurrence of the broad sideband pattern in-
dicates that the aniline molecules are rigidly bound to the
zeolite framework. The signal at ca. 129 ppm corresponds
to aniline carbon atoms in meta positions and is very close
to the corresponding value in solution (129.1 ppm (22)). In
contrast, the signal at ca. 122 ppm assigned to carbon atoms
in ortho and para positions is shifted significantly to lower
field (solution data: 114.4 and 116.3 ppm, respectively (22)).
This result suggests that, upon aniline adsorption, Brønsted
acid sites of zeolite H–Y interact with the NH2 groups of
aniline molecules, causing a significant distortion of the
carbon atoms in ortho and para positions and points to a
partial protonation of the aniline molecules adsorbed on
zeolite H–Y.
13C MAS NMR measurements were carried out on an
MSL-300 Bruker spectrometer operating at 75.15 MHz.
Conditions allowing a quantitative evaluation were achie-
ved using high-power gated proton decoupling with sup-
pressed nuclear Overhauser effect. Experiments with cross-
TABLE 1
The 13C MAS NMR spectrum observed after adsorption
of methanol-13C shows two resonances: a broad and weak
signal at ca. 56 ppm and a narrow intense line at ca. 49 ppm.
The narrow line at 49 ppm with liquid-like characteristics
is attributed to mobile methanol species, which are most
probably attached to bridging hydroxyls, as was shown in
(23). The broad signal at 56 ppm corresponding to more
rigid species, which are amenable to cross-polarization, is
assigned to surface methoxy groups. The chemical shift and
the behavior of the latter species are consistent with previ-
ous observations for surface methoxy groups obtained by
reaction with methyl iodide over zeolite H–Y (24, 25).
Co-adsorption of aniline and methanol-13C results in
the appearance of a new resonance line at ca. 51 ppm
Catalyst Samples Prepared in Sealed Glass Cells for the
Controlled-Atmosphere13C MASNMR Investigationsunder Batch
Conditions
Loaded reactants (in molecules/u.c.)
Catalyst
Methanol-13
C
Methyl iodide-13
C
Aniline
H–Y
H–Y
H–Y
H–Y
—
7
7
—
—
—
—
—
15
—
21
—
21
—
21
H–Y, methoxylated
by CH3OH-13
H–Y, methoxylated
by CH3I-13
C
—
—
10
C

NMR MECHANISTIC STUDY OF ANILINE METHYLATION
377
FIG. 1. 13C MAS NMR and 13C CP/MAS NMR spectra recorded after adsorption of aniline, methanol-13C, and a mixture of methanol-13C and
aniline on zeolite H–Y at 293 K. Asterisks indicate spinning sidebands.
corresponding to rigid species as evidenced by the experi- 2. Aniline Alkylation with Methanol
ment with cross-polarization. This line could be caused by
methanol interacting with aniline, which is H-bonded to
Brønsted acid sites of zeolite H–Y (aniline/methanol ad- line with methanol on zeolite H–Y are presented in Fig. 2.
sorption complex).
Alkylation of aniline starts at 373 K, as evidenced by the
13C MAS NMR spectra recorded after reaction of ani-
To conclude, the results of NMR spectroscopy give the appearance of a small signal at 38.5 ppm attributed to the
following picture of aniline and methanol adsorption on ze- methyl groups of N-methylaniline. This assignment was
olite H–Y. Aniline, which is more strongly adsorbed than confirmed by adsorption of pure N-methylaniline on ze-
methanol, is supposed to be H-bonded to Brønsted acid olite H–Y. The significant shift of the line corresponding to
sites of zeolite H–Y. As a result, positive charge is partially N-methylaniline with respect to solution data (29.9 ppm
transferred to the NH2 group of the aniline molecule, lead- (22)) is due to a strong interaction between the basic–
ing to the formation of a surface aniline complex. A small NH–groups of the N-methylaniline molecules and zeolite
part of the methanol molecules (ca. 15% ) also interacts with Brønsted acid sites, leading to a significant disturbance of
Brønsted acid sites of the zeolite to form surface methoxy the methyl groups. The appearence of a new NMR signal
groups; another part (ca. 55% ) gives adsorption com- at 38.5 ppm is accompanied by the increase of the concen-
plexes with H-bonded aniline molecules and the remain- tration of methoxy species from 15% up to 45% and by
der (ca. 30% ) is present as methanol molecules with high merging of the lines at 49 and 51 ppm into a very broad
mobility.
signal centered at ca. 49 ppm. This merger is due to the

378
IVANOVA ET AL.
is observed only at reaction temperatures from 523 to
623 K after complete conversion of methanol species into
N-methylaniline. It is hence concluded that toluidines and
N-methyltoluidines are formed via isomerization or dispro-
portionation of N-methylaniline.
3. Interaction of Aniline with Methoxylated Zeolite H–Y
To confirm the intermediate role of surface methoxy
groups in aniline alkylation with methanol, the methoxy-
lation of zeolite H–Y with methanol and methyl iodide
and the interaction of aniline with the methoxylated zeolite
were studied. The corresponding 13C MAS NMR spectra
are presented in Figs. 3 and 4.
As discussed in section 1, adsorption of methanol on
evacuated zeolite H–Y leads to the formation of surface
methoxy groups already at ambient temperature. However,
the concentration of methoxy groups at this temperature is
rather low. Therefore, the sample with adsorbed methanol
was heated in a stepwise manner to find the optimal tem-
perature for the formation of methoxy groups (Fig. 3a). The
spectra show that the maximum intensity of the signal due
to methoxy groups occurs at 438 K. Further heating leads to
the formation of dimethyl ether (59 ppm) and finally results
in the conversion of methanol to hydrocarbons evidenced
by the appearance of a large number of lines in the spectral
region between 0 and 35 ppm.
The 13C MAS NMR spectra obtained after methoxyla-
tion of zeolite H–Y with methyl iodide are shown in Fig. 3b.
With this reactant, the reaction is much more difficult and
only a small part of CH3I is converted to surface methoxy
FIG. 2. 13C MAS NMR spectra recorded in the course of reaction be-
tween methanol-13C and aniline on zeolite H–Y. Asterisks indicate spin-
ning sidebands.
decrease of the contribution of the line at 51 ppm caused groups, even at elevated temperatures. The optimal tem-
by a destruction of a part of the aniline adsorption com- perature of methoxylation with CH3I is around 430 to
plexes and removal of the corresponding aniline molecules 450 K, as in the case of methoxylation by methanol. At
from Brønsted acid sites into the gas phase by the more higher temperatures, the methoxy groups are converted
stable surface methoxy groups.
into hydrocarbons. The possibility to remove unreacted
At a temperature of 423 K, the line at 56 ppm corre- methyl iodide was checked in a separate experiment in
sponding to surface methoxy groups yields the highest con- which the sample loaded with methyl iodide was first
tribution to the NMR spectrum. The concentration of N- heated to 433 K for 5 h and then evacuated at 393 K.
methylaniline at 38.5 ppm also increases. In addition, two The spectrum presented in Fig. 3b, top, shows a partial
small resonance lines at ca. 59 and 63 ppm appear, which surface methoxylation (signal at 56 ppm) and satisfactory
are due to different forms of adsorbed dimethyl ether (26). removal of the unreacted reagent (signal at 23 ppm).
Further heating to 473 K leads to a disappearance of the Therefore, the latter conditions were selected for a
signal of surface methoxy groups followed by a rapid for- methoxylation of the zeolite surface with both methanol
mation of N-methylaniline. This observation suggests that and methyl iodide.
methoxy groups may play a role as intermediates in the
N-alkylation on zeolite H–Y.
After methoxylation, the samples were further sub-
jected to aniline adsorption at ambient temperature. It is
Finally, at a temperature of 523 K, all 13C-labeled species interesting to note that N-alkylation, evidenced by the ap-
are converted into methyl groups of N-methylaniline pearance of the line at 38.5 ppm, is observed already at
molecules. Consequently, N-methylaniline is the only pri- this temperature (Fig. 4). Further heating at temperatures
mary alkylation product on zeolite H–Y, and surface from 373 to 473 K leads to a complete transformation of the
methoxy groups are suggested to be intermediates in this surface methoxy groups into N-methylaniline. It should be
reaction.
mentioned that, in the case of zeolite H–Y methoxylated
Formation of the secondary reaction products, viz. to- with methanol, the reaction with aniline is slower since the
luidines and N-methyltoluidines, evidenced by the ap- evacuation procedure did not allow removal of all unre-
pearance of 13C NMR signals at 17 to 24 ppm (Fig. 2), acted methanol and water formed during methoxylation.

FIG. 3. 13C MAS NMR spectra recorded during reactions of methanol-13C (a) and methyl iodide-13C (b) on zeolite H–Y. Asterisks indicate spinning
sidebands.
FIG. 4. 13C MAS NMR spectra recorded during reaction of aniline on zeolite H–Y methoxylated by methanol-13C (a) and methyl iodide-13C (b).
Zeolite methoxylation was carried out prior to aniline adsoprtion by treatment with 13C labeled CH3OH (a) or CH3I (b) at 433 K for 5 h followed by
evacuation at 393 K to remove unreacted methoxylating agent. Asterisks indicate spinning sidebands.
379

380
IVANOVA ET AL.
FIG. 5. Mechanism proposed for aniline alkylation with methanol on zeolite H–Y.
In conclusion, the results obtained confirm the hypoth-
CONCLUSIONS
esis on the intermediate role of methoxy species in N-
alkylation. Moreover, the in situ 13C MAS NMR investiga-
tions demonstrated that the formation of methoxy species
is the limiting step of the overall reaction.
Co-adsorption of aniline and methanol on zeolite H–Y
leads to strongly adsorbed aniline molecules and three
types of methanol species with different mobility: (i) mo-
bile methanol molecules adsorbed on bridging hydroxyls,
(ii) methanol molecules with limited mobility due to an in-
teraction with H-bonded aniline, and (iii) a small amount
of methoxy groups strongly bound to the zeolite surface.
N-Methylaniline is the primary product of aniline alky-
lation. Surface methoxy groups play a role of intermediate
species in N-alkylation. The limiting step of the reaction is
the formation of surface methoxy groups.
4. Mechanism of the Methylation of Aniline by Methanol
The reaction mechanism proposed in Fig. 5 rationalizes
the experimental results described in sections 1 to 3.
Upon adsorption, the aniline molecules are H-bonded to
the Brønsted acid sites of zeolite H–Y and occupy most of
these sites. A small part of methanol molecules reacts with
free Brønsted acid sites to form surface methoxy groups.
Another part gives adsorbate complexes with H-bonded
aniline molecules, and the remainder is present as methanol
molecules with high mobility.
Formation of toluidines and N-methyltoluidines occurs
via secondary isomerization or disproportionation of N-
methylaniline.
In the first step, methanol reacts with the zeolite surface
to give methoxy groups. This reaction may occur via two re-
action pathways, namely, a direct interaction of methanol
with free Brønsted acid sites or an interaction of methanol
with H-bonded aniline molecules. The first reaction path-
way was thoroughly investigated by quantum chemical
calculations (27, 28) and was suggested to proceed via a
methoxonium-like ion-pair complex (six-membered tran-
sition state). The second pathway may occur via a sim-
ilar mechanism including an eight-membered transition
state (Fig. 5). As a result of this step, a part of the ani-
line molecules is displaced from the Brønsted acid sites
into the gas phase by the more stable surface methoxy
groups.
In the second step of the reaction, aniline molecules re-
act with surface methoxy groups to give N-methylaniline.
The latter reaction may occur via a concerted mechanism
involving a six-membered transition state, as shown in the
right part of Fig. 5.
The experimental data suggest that the second reaction
step is much faster than the first one and that the formation
of surface methoxy groups is the limiting step of the overall
reaction.
ACKNOWLEDGMENTS
Financial support by the Volkswagen Foundation is gratefully acknowl-
edged. I. I. Ivanova gives thanks for a President grant of the Russian
Federation. E. B. Pomakhina thanks Haldor Topsoe AS for a postgraduate
fellowship. J. Weitkamp and M. Hunger acknowledge financial support
by Deutsche Forschungsgemeinschaft, Max-Buchner-Forschungsstiftung,
and Fonds der Chemischen Industrie.
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