Formation and decomposition of N,N,N-trimethylanilinium cations on zeolite H-Y investigated by in situ stopped-flow MAS NMR spectroscopy

Article abstract of DOI:10.1021/ja012675n

Methylation of aniline by methanol on zeolite H-Y has been investigated by in situ ¹³C MAS NMR spectroscopy under flow conditions. The in situ ¹³C continuous-flow (CF) MAS NMR experiments were performed at reaction temperatures between 473 and 523 K, molar methanol-to-aniline ratios of 1:1 to 4:1, and modified residence times of ¹³CH₃OH between 20 and 100 (g-h)/mol. The methylation reaction was shown to start at 473 K. N,N,N-Trimethylanilinium cations causing a ¹³C NMR signal at 58 ppm constitute the major product on the catalyst surface. Small amounts of protonated N-methylaniline ([PhNH₂CH₃]⁺) and N,N-dimethylaniline ([PhNH(CH₃)₂]₊) were also observed at ca. 39 and 48 ppm, respectively. After increase of the temperature to 523 K, the contents of N,N-dimethylanilinium cations and ring-alkylated reaction products strongly increased, accompanied by a decrease of the amount of N, N,N-trimethylanilinium cations. With application of the in situ stopped-flow (SF) MAS NMR technique, the decomposition of N,N,N-trimethylanilinium cations on zeolite H-Y to N,N-dimethylanilinium and N-methylanilinium cations was investigated to gain a deeper insight into the reaction mechanism. The results obtained allow the proposal of a mechanism consisting of three steps: (i) the conversion of methanol to surface methoxy groups and dimethyl ether (DME); (ii) the alkylation of aniline with methanol, methoxy groups, or DME leading to an equilibrium mixture of N,N,N-trimethylanilinium, N,N-dimethylanilinium, and N-methylanilinium cations attached to the zeolite surface; (iii) the deprotonation of N,N-dimethylanilinium and N-methylanilinium cations causing the formation of N,N-dimethylaniline (NNDMA) and N-methylaniline (NMA) in the gas phase, respectively. The chemical equilibrium between the anilinium cations carrying different numbers of methyl groups is suggested to play a key role for the products distribution in the gas phase.

Full text of DOI:10.1021/ja012675n

Published on Web 06/01/2002
Formation and Decomposition of N,N,N-Trimethylanilinium
Cations on Zeolite H-Y Investigated by in Situ Stopped-Flow
MAS NMR Spectroscopy
Wei Wang,*,†,‡ Michael Seiler,† Irina I. Ivanova,§ Ulrich Sternberg,|
Jens Weitkamp,† and Michael Hunger*,†
Contribution from the Institute of Chemical Technology, UniVersity of Stuttgart,
D-70550 Stuttgart, Germany, Department of Chemistry, Moscow State UniVersity, Leninskie
Gory, 119899 Moscow, Russia, and Faculty of Physics and Astronomy, Friedrich Schiller
UniVersity of Jena, Max-Wien-Platz 1, D-07743 Jena, Germany
Received December 7, 2001. Revised Manuscript Received February 27, 2002
Abstract: Methylation of aniline by methanol on zeolite H-Y has been investigated by in situ 13C MAS
NMR spectroscopy under flow conditions. The in situ 13C continuous-flow (CF) MAS NMR experiments
were performed at reaction temperatures between 473 and 523 K, molar methanol-to-aniline ratios of 1:1
to 4:1, and modified residence times of 13CH3OH between 20 and 100 (g‚h)/mol. The methylation reaction
was shown to start at 473 K. N,N,N-Trimethylanilinium cations causing a 13C NMR signal at 58 ppm constitute
the major product on the catalyst surface. Small amounts of protonated N-methylaniline ([PhNH2CH3]+)
and N,N-dimethylaniline ([PhNH(CH3)2]+) were also observed at ca. 39 and 48 ppm, respectively. After
increase of the temperature to 523 K, the contents of N,N-dimethylanilinium cations and ring-alkylated
reaction products strongly increased, accompanied by a decrease of the amount of N,N,N-trimethylanilinium
cations. With application of the in situ stopped-flow (SF) MAS NMR technique, the decomposition of N,N,N-
trimethylanilinium cations on zeolite H-Y to N,N-dimethylanilinium and N-methylanilinium cations was
investigated to gain a deeper insight into the reaction mechanism. The results obtained allow the proposal
of a mechanism consisting of three steps: (i) the conversion of methanol to surface methoxy groups and
dimethyl ether (DME); (ii) the alkylation of aniline with methanol, methoxy groups, or DME leading to an
equilibrium mixture of N,N,N-trimethylanilinium, N,N-dimethylanilinium, and N-methylanilinium cations
attached to the zeolite surface; (iii) the deprotonation of N,N-dimethylanilinium and N-methylanilinium cations
causing the formation of N,N-dimethylaniline (NNDMA) and N-methylaniline (NMA) in the gas phase,
respectively. The chemical equilibrium between the anilinium cations carrying different numbers of methyl
groups is suggested to play a key role for the products distribution in the gas phase.
Introduction
investigation of the formation and transformation of surface
compounds under steady-state conditions and, in some cases, a
Since 1995, a number of new in situ MAS NMR techniques
under continuous-flow (CF) conditions have been developed
for the study of heterogeneously catalyzed reactions under
industrial conditions.1-6 With these techniques, a direct NMR
simultaneous gas chromatographic analysis of the reaction
products are possible.1 Using an isolated flow variable-temper-
ature (VT) MAS NMR probe, Munson and co-workers have
been able to study the conversion of methanol to dimethyl ether
on zeolite HZSM-5 in situ and found that there is no equilibrium
between methanol and dimethyl ether in the catalyst at high
temperatures under flow conditions.2 Inspired by the spectro-
scopic study of enzyme catalytic reactions, Haw and co-workers3
have exploited an external microreactor to freeze the reaction
system by a rapid quench technique and obtained the snapshot
of the working catalysts after transferring them into the MAS
* To whom correspondence should be addressed. Phone: +49/711/685-
4079. Fax: +49/711/685-4081. E-mail: michael.hunger@po.uni-stuttgart.de.
† University of Stuttgart.
‡ Current address: Loker Hydrocarbon Research Institute and Department
of Chemistry, University of Southern California, Los Angeles, CA 90089-
1661. E-mail: wangwei@rcf.usc.edu.
§ Moscow State University.
| Friedrich Schiller University of Jena.
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J. AM. CHEM. SOC. 2002, 124, 7548-7554
10.1021/ja012675n CCC: $22.00 © 2002 American Chemical Society

N,N,N-Trimethylanilinium Cations on Zeolite H−Y
A R T I C L E S
rotor and NMR spectrometer. Recently, continuous-flow MAS
NMR techniques have been also successfully combined with
an optical pumping process to deliver laser-polarized xenon
allowing a selective coherence transfer to nuclei located at the
surface of solid materials and a sophisticated characterization
of microporous solids by 129Xe NMR spectroscopy.7-10
On the basis of in situ CF MAS NMR spectroscopy, we
introduced very recently a new stopped-flow (SF) technique,11,12
which possesses a high potential for determining intermediates
and elucidating the mechanisms of a broad variety of hetero-
geneously catalyzed reactions. The main feature of this method
is a consecutive in situ MAS NMR investigation of the working
catalyst under flow conditions by stopping the reactant flow
and obserVing the further transformation of adsorbed com-
pounds at reaction temperatures. By comparing the in situ MAS
NMR spectra obtained under CF and SF conditions, it is possible
to unambiguously distinguish intermediates from reaction
products, thus allowing a safer identification of the reaction
mechanisms involved. Following this approach, N-methylene-
aniline was identified for the first time as an intermediate in
the methylation of aniline on a basic zeolite CsOH/Cs,Na-Y.11
Furthermore, a variant of the new in situ stopped-flow MAS
NMR technique was introduced, the salient feature of which is
the application of a purging period.12 During the purging period,
surface compounds such as methoxy groups formed on the
catalyst were selectiVely retained, and the role of methoxy
groups in the catalytic formation of dimethyl ether (DME) has
been shown.12
methanol-to-aniline ratio as well as by the nature of the catalyst.
Up until now, however, the reaction was studied almost
exclusively by analyzing the product distribution in the gas phase
using gas chromatography. No direct observation of the working
catalyst, which is desirable for the formulation of more reliable
and more sophisticated reaction mechanisms, has so far been
achieved. Recently, in situ 13C MAS NMR investigations of
aniline methylation on zeolite H-Y under batch conditions using
samples sealed in glass ampules were performed for the first
time.17 With a molar methanol-to-aniline ratio of 1:3 at reaction
temperatures between 373 and 523 K, the primary alkylation
product was N-methylaniline (NMA), and the formation of
surface methoxy groups was found to be the rate-limiting step
of the overall reaction.
In this work, we report on in situ 13C MAS NMR investiga-
tions of the methylation of aniline by methanol on acidic zeolite
H-Y under flow conditions. With application of the in situ
stopped-flow (SF) MAS NMR technique, the formation and
decomposition of N,N,N-trimethylanilinium cations on zeolite
H-Y were studied, and unequivocal experimental evidence for
the decomposition of N,N,N-trimethylanilinium cations into N,N-
dimethylanilinium and N-methylanilinium cations was obtained.
Moreover, a mechanism of catalytic aniline methylation, in
which the equilibrium between N,N,N-trimethylanilinium, N,N-
dimethylanilinium, and N-methylanilinium cations on the cata-
lyst surface plays an important role, is proposed and discussed.
Experimental Section
Although stopped-flow spectroscopic techniques are well
established in many fields of chemistry, such as for NMR
investigations of the real-time refolding of proteins,13 the
application of the in situ SF MAS NMR technique in hetero-
geneous catalysis is entirely new. In this work, this technique
is utilized for an investigation of aniline methylation on acidic
zeolite H-Y. As an industrially important process for the
manufacture of raw materials for organic syntheses and inter-
mediates, aniline methylation by methanol has been performed
on a number of different catalysts.14-16 The products were found
to be N-methylaniline (NMA), N,N-dimethylaniline (NNDMA),
and toluidines. The product distribution is influenced by the
reaction temperature, the modified residence time, and the molar
Materials. Zeolite Na-Y (nSi/nAl ) 2.7) was purchased from
Degussa AG, Hanau, Germany. The ammonium form (NH4-Y) was
prepared by a 4-fold ion exchange of Na-Y at 353 K in a 1.0 M
aqueous solution of NH4NO3.18 When an ion exchange degree of 92%
was reached, the material was washed in deionized water and dried at
room temperature. Subsequently, NH4-Y was heated in a vacuum with
a rate of 20 K/h up to the final temperature of 673 K. There, the material
was calcined at a pressure below 10-2 Pa for 12 h leading to zeolite
H-Y. Zeolite H-Y was characterized by AES-ICP, XRD, and solid-
1
state H, 27Al, and 29Si MAS NMR spectroscopy which indicated that
the material obtained after cation exchange and calcination was neither
damaged nor dealuminated.
Methanol-13C (13C-enrichment 99%) and methyl-13C iodide (13C-
enrichment 99%) were obtained from Cambridge Isotopes. Aniline
(>99.5%) was purchased from Fluka. Dimethyl ether (DME, >99%),
N-methylaniline (NMA, >99.5%), N,N-dimethylanline (NNDMA,
>99%), and ortho- (>99.5%) and para-toluidines (> 99.5%), all with
a natural abundance of 13C isotopes, were obtained from Aldrich.
In Situ Continuous-Flow (CF) MAS NMR Experiments. Prior
to the in situ CF MAS NMR experiments, 250 mg of calcined zeolite
H-Y was filled into a 7 mm MAS NMR rotor reactor under dry
nitrogen in a glovebox and pressed to a cylindrical catalyst bed. After
transfer of the rotor into the high-temperature Doty MAS NMR probe,
a second in situ dehydration of the catalyst material was performed at
673 K for 1 h under flowing nitrogen (30 mL/min). During the in situ
MAS NMR experiments under continuous-flow conditions at temper-
atures between 298 and 523 K, carrier gas (dry nitrogen) loaded with
13CH3OH and aniline was injected into the MAS NMR rotor reactor
by applying the equipment described elsewhere.1b In different experi-
ments, a modified residence time, W/F, of 13CH3OH between 20 and
(7) (a) Haake, M.; Pines, A.; Reimer, J. A.; Seydoux, R. J. Am. Chem. Soc.
1997, 119, 11711-11712. (b) Brunner, E.; Seydoux, R.; Haake, M.; Pines,
A.; Reimer, J. A. J. Magn. Reson. 1998, 130, 145-148.
(8) (a) Moudrakovski, I. L.; Nossov, A. V.; Lang, S.; Breeze, S. R.; Ratcliffe,
C. I.; Simard, B.; Santyr, G.; Ripmeester, J. A. Chem. Mater. 2000, 12,
1181-1183. (b) Nossov, A. V.; Soldatov, D. V.; Ripmeester, J. A. J. Am.
Chem. Soc. 2001, 123, 3563-3568.
(9) Smith, L. J.; Smith, J.; MacNamara, E.; Knagge, K.; Raftery, D. J. Phys.
Chem. B 2001, 105, 1412-1421.
(10) (a) Kneller, J. M.; Soto, R. J.; Surber, S. E.; Colomer, J. F.; Fonseca, A.;
Nagy, J. B.; Pietrass, T. J. Magn. Reson. 2000, 147, 261-265. (b) Kneller,
J. M.; Soto, R. J.; Surber, S. E.; Colomer, J. F.; Fonseca, A.; Nagy, J. B.;
van Tendeloo, G.; Pietrass, T. J. Am. Chem. Soc. 2000, 122, 10591-10597.
(11) Wang, W.; Seiler, M.; Ivanova, I. I.; Weitkamp, J.; Hunger, M. Chem.
Commun. 2001, 1362-1363.
(12) Wang, W.; Seiler, M.; Hunger, M. J. Phys. Chem. B 2001, 105, 12553-
12558.
(13) (a) Hoeltzli, S. D.; Frieden, C. Biochemistry 1996, 35, 16843-16851. (b)
Hoeltzli, S. D.; Frieden, C. Biochemistry 1998, 37, 387-398.
(14) (a) Bautista, F. M.; Campelo, J. M.; Garcia, A.; Luna, D.; Marinas, J. M.;
Romero, A. A.; Urbano, M. R. J. Catal. 1997, 172, 103-109. (b) Bautista,
F. M.; Campelo, J. M.; Garcia, A.; Luna, D.; Marinas, J. M.; Romero, A.
A. Stud. Surf. Sci. Catal. 1997, 108, 123-130. (c) Bautista, F. M.; Campelo,
J. M.; Garcia, A.; Luna, D.; Marinas, J. M.; Romero, A. A. Appl. Catal.,
A 1998, 166, 39-45.
(17) (a) Ivanova, I. I.; Pomakhina, E. B.; Rebrov, A. I.; Kolyagin, Y. G.; Hunger,
M.; Weitkamp, J. Stud. Surf. Sci. Catal. 2001, 135, 23-P-12. (b) Ivanova,
I. I.; Pomakhina, E. B.; Rebrov, A. I.; Hunger, M.; Kolyagin, Y. G.;
Weitkamp, J. J. Catal. 2001, 203, 375-381.
(15) Shaikh, R. A.; Singh, P. S.; Bandyopadhyay, R.; Kavedia, C. V.; Mirajkar,
S. P.; Rao, B. S. Stud. Surf. Sci. Catal. 1998, 113, 637-642.
(16) Narayanan, S.; Deshpande, K. Appl. Catal., A 2000, 199, 1-31 and
references therein.
(18) Hunger, M.; Horvath, T. Catal. Lett. 1997, 49, 95-100.
9
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A R T I C L E S
Wang et al.
Figure 1. Protocol of the in situ stopped-flow (SF) MAS NMR experiment
consisting of periods i-iii specified in the text, allowing the study of
heterogeneously catalyzed reactions under steady-state conditions and of
the progressive reaction of adsorbates previously formed during period i.
100 (g‚h)/mol was used, and the molar 13CH3OH to aniline ratio was
varied from 1:1 to 4:1.
In Situ Stopped-Flow (SF) MAS NMR Experiments. The stopped-
flow experiment applied in this work consists of the following steps
(Figure 1): (i) recording of the in situ MAS NMR spectra at reaction
temperatures and under steady-state conditions during a continuous flow
of the reactants into the MAS NMR rotor reactor; (ii) recording of
MAS spectra at room temperature after stopping the flow of reactants
and purging the catalyst with dry nitrogen, which is different from the
protocol used in ref 11 and, in some cases,12 has the advantage of
selectively removing reactants from the catalyst prior to the subsequent
investigation of intermediates; (iii) recording of MAS NMR spectra
after raising the temperature to reaction conditions without starting the
reactant flow. By comparison of the NMR spectra recorded before and
after stopping the reactant flow, detailed information on the progressive
reaction of adsorbates formed in period i can be collected.11,12
Experiments with Externally Loaded Samples. Experiments with
externally loaded samples were mainly performed to confirm the
assignments of the 13C NMR signals of reaction products observed by
in situ MAS NMR under flow conditions. Different chemical species,
such as N-methylaniline (NMA), N,N-dimethylaniline (NNDMA),
ortho-toluidine, para-toluidine, and mixtures of DME and aniline, were
adsorbed at different partial pressures on calcined zeolite H-Y which
had been filled into an MAS NMR rotor in advance. Prior to the loading,
several “freeze-pump cycles” were carried out to purify the adsorptives.
During the external loading, the rotor was connected to a vacuum line.
Subsequently, the MAS NMR rotor was sealed off and the MAS NMR
spectra were recorded at ambient temperature. To prepare a sample
loaded with a mixture of DME and aniline, the zeolite was filled into
a glass tube and calcined as mentioned above. The adsorption of the
reactants was performed in the vacuum line by cooling the glass tube
with liquid nitrogen. Subsequently, the glass tube was rapidly fused
under liquid nitrogen and carefully heated to a desired reaction
temperature. Prior to the NMR measurements, the samples were filled
into a gastight MAS NMR rotor under dry nitrogen.
NMR Spectroscopy. 13C MAS NMR investigations were performed
on a Bruker MSL 400 spectrometer at a resonance frequency of 100.6
MHz. CF and SF MAS NMR experiments were carried out with a
sample spinning rate of ca. 2.0 kHz using a modified DSI-740 7 mm
STD MAS NB NMR probe of Doty Scientific Instruments, TX (see
ref 1c). 13C MAS NMR investigations under batch conditions with
externally loaded samples were performed with a sample spinning rate
of ca. 3.5 kHz using a 7 mm Bruker MAS NMR probe at ambient
temperature. In situ 13C CF MAS NMR and SF MAS NMR spectra
were recorded at reaction temperature with high-power proton decou-
pling after an excitation with π/2 pulses and a repetition time of 5 s.
13C CP/MAS NMR experiments (CP: cross-polarization) were carried
out at reaction temperature with a contact time of 2.5 ms and a repetition
time of 4 s. The number of scans of 200-500 was optimized depending
on the signal/noise ratio obtained at different temperatures. To record
13C MAS NMR spectra of externally loaded samples (natural abundance
Figure 2. In situ 13C MAS NMR spectra recorded during aniline
methylation on zeolite H-Y under continuous-flow conditions (W/F ) 40
(g‚h)/mol, 13CH3OH/aniline ) 2:1) at reaction temperatures of 473 K for
10 min (a), 473 K for 90 min (b), 498 K for 90 min (c), and 523 K for 90
min (d).
of 13C isotopes), excitations with π/2 pulses, high-power proton
decoupling, and a repetition time of 5 s were used. The typical number
of scans was between 16 000 and 20 000. All spectra were referenced
to tetramethylsilane (TMS).
Results and Discussion
Formation of N,N,N-Trimethylanilinium Cations. Figure
2 shows the in situ 13C CF MAS NMR spectra recorded during
methylation of aniline by methanol at reaction temperatures of
473-523 K. During these experiments, a mixture of 13CH3OH
(W/F ) 40 (g‚h)/mol) and aniline in a molar ratio of 2:1 was
continuously injected into the MAS NMR rotor reactor. The
signal occurring at 50 ppm in the spectrum recorded at 473 K
(Figure 2a) is caused by methanol adsorbed on zeolite H-Y.
The signals occurring at 63.5 and 60.5 ppm are due to dimethyl
ether (DME) formed from methanol. According to Ivanova and
Corma19 and Blaszkowski and van Santen,20 these two signals
are attributed to side-on and end-on adsorbate conformations
of dimethyl ether, respectively, in agreement with the stronger
adsorption of the species occurring at 63.5 ppm.12
After a reaction time of 90 min at 473 K, a signal of
N-methylanilinium cations ([PhNH2CH3]+) appears at 39 ppm
(Figure 2b), which were formed on the Brønsted acid sites of
zeolite H-Y. The chemical shift of the N-methylanilinium
cations was confirmed by loading pure NMA on zeolite H-Y
indicating a strong low-field shift of the methyl signal of NMA
due to a protonation by strong Brønsted acid sites (vide infra).
In addition, another signal occurs at 58 ppm (Figure 2b). Upon
further increase of the reaction temperature, the intensity of this
(19) Ivanova, I. I.; Corma, A. J. Phys. Chem. B 1997, 101, 547-551.
(20) Blaszkowski, S. R.; van Santen, R. A. J. Phys. Chem. B 1997, 101, 2292-
2305.
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N,N,N-Trimethylanilinium Cations on Zeolite H−Y
A R T I C L E S
Figure 3. In situ 13C MAS NMR spectra of zeolite H-Y recorded at
temperatures of 433 K (a) and 473 K (b) during a continuous injection of
13CH3OH (W/F ) 40 (g‚h)/mol) into the MAS NMR rotor reactor. The
spectra on the left-hand side were obtained by MAS NMR and proton
decoupling, while the spectra on the right-hand side were recorded applying
CP/MAS NMR (cross polarization). Asterisks denote spinning sidebands.
signal increases, while the intensities of the methanol signal at
50 ppm and of the DME signals at 60.5 and 63.5 ppm decrease
and eventually disappear (Figure 2c,d). During aniline methyl-
ation at reaction temperatures of 498-523 K, additional signals
appear at 48, 21, and 16 ppm (Figure 2c,d). The signal at 48
ppm, dominating in Figure 2d, is due to N,N-dimethylanilinium
cations ([PhNH(CH3)2]+), while the signals at 16 and 21 ppm
originate from ring-alkylated reaction products, i.e., by anilines
(ortho- and para-toluidine) or anilinium cations with methyl
groups in ortho and para positions. These assignments have
been confirmed by an external loading of the individual
compounds onto zeolite H-Y.
Figure 4. 13C CP/MAS NMR spectra (CP: cross polarization) recorded
during aniline methylation on zeolite H-Y under continuous-flow conditions
(W/F ) 40 (g‚h)/mol, 13CH3OH/aniline ) 2:1) at temperatures between
298 and 473 K. Asterisks denote spinning sidebands.
Table 1. 13C Chemical Shift and Proton Affinity (PA) of the
Species Absorbed on Zeolite H-Y during Aniline Methylation by
Methanol
Since the resonance position at 58 ppm may be indicative of
carbon atoms bound to oxygen, the formation of surface
methoxy groups (CH3OZ, Z denotes the zeolite framework)17,21
was considered in more detail. However, an assignment of the
signal at 58 ppm to surface methoxy groups could be ruled out
for the following reasons: (i) The conversion of pure methanol
on zeolite H-Y under the same continuous-flow conditions gave
no 13C MAS NMR signal at 58 ppm (Figure 3, left). A
simultaneous CP/MAS NMR experiment which favors signals
arising from rigidly bound species led to a signal with a
resonance position of 56.2 ppm which is characteristic for
methoxy groups formed on zeolite H-Y (Figure 3, right). (ii)
In situ 13C CP/MAS NMR spectra recorded during the conver-
sion of mixtures of 13CH3OH and aniline consist of two separate
signals, a narrow line at 58 ppm and a broad signal at 56.2
ppm with spinning sidebands corresponding to surface methoxy
groups (Figure 4).
13C chem shift proton affinityb
(ppm)
(kJ/mol)
methanol
50
754.3
792.0
dimethyl ether (DME)
methoxy groups (CH3OZ)
anilinea
60.5, 63.5
56.2
882.5
916.6c
941.1d
N-methylanilinium cations, [PhNH2CH3]+
39
N,N-dimethylanilinium cations, [PhNH(CH3)2]+ 48
N,N,N-trimethylanilinium cations, [PhN(CH3)3]+ 58
ortho-toluidinea
para-toluidinea
16
21
890.9
896.7
a Probably also protonated on zeolite H-Y; see the text. b Data obtained
from NIST Chemistry WebBook (http://webbook.nist.gov/chemistry). c PA
value of the parent compound, N-methylaniline (NMA). d PA value of the
parent compound, N,N-dimethylaniline (NNDMA).
by the observation of a quartet which splits up after releasing
the proton decoupling. An origination of the signal at 58 ppm
from hydrogen-bonded adsorbate complexes, such as methanol
or dimethyl ether interacting with aniline, could be clearly
excluded by molecular dynamic calculations.24 The assignments
of all 13C NMR signals in this work are listed in Table 1.
The 13C CF MAS NMR spectra in Figure 2 indicate that, at
temperatures between 473 and 498 K, N,N,N-trimethylanilinium
cations are formed as the major reaction product on the catalyst
surface, accompanied by a decrease of the signals of methanol
and DME. At 523 K, however, the intensities of the 13C NMR
signals of N,N-dimethylanilinium (48 ppm) and N-methyl-
anilinium cations (39 ppm) and of para- (21 ppm) and ortho-
methylated products (16 ppm) increase, while the intensity of
the signal of N,N,N-trimethylanilinium cations (58 ppm) de-
The 13C MAS NMR signal at 58 ppm was, therefore,
attributed to N,N,N-trimethylanilinium cations ([PhN(CH3)3]+)
formed on zeolite H-Y, on the basis of the result of the
conversion of methanol and aniline. This assignment is strongly
supported by the resonance position of 57 ppm for the signal
of tetramethylammonium cations, (CH3)4N+, observed by
Thursfield et al.22 investigating the reaction of methanol and
ammonia on acidic zeolites H-Rho and H-SAPO-34 by 13C
MAS NMR spectroscopy under batch conditions. Studying the
same reaction, Ernst and Pfeifer23 clearly assigned a narrow 13
C
NMR signal at ca. 56 ppm to tetramethylammonium cations
(21) Bosacek, V.; Klik, R.; Genoni, F.; Spano, G.; Rivetti, F.; Figueras, F. Magn.
Reson. Chem. 1999, 37, S135-S141.
(22) Thursfield, A.; Anderson, M. W.; Dwyer, J.; Hutchings, G. J.; Lee, D. J.
Chem. Soc., Faraday Trans. 1998, 94, 1119-1122.
(24) Sternberg, U. Unpublished results. Available at Friedrich Schiller University
of Jena upon request.
(23) Ernst, H.; Pfeifer, H. J. Catal. 1992, 136, 202-208.
9
J. AM. CHEM. SOC. VOL. 124, NO. 25, 2002 7551

A R T I C L E S
Wang et al.
creases. Various in situ 13C CF MAS NMR experiments were
performed with different modified residence times of 13CH3OH
(W/F ) 20, 40, 75, and 100 (g‚h)/mol) and molar 13CH3OH to
aniline ratios (1:1, 2:1, and 4:1). In some experiments, the in
situ 13C CF MAS NMR spectra were recorded after raising the
reaction temperature directly to the desired temperature of 473,
498, 523, or 573 K rather than by increasing it in steps of 25
K. In each case, the same result was obtained: at low reaction
temperatures, N,N,N-trimethylanilinium cations were formed as
the predominant product, while N,N-dimethylanilinium and
N-methylanilinium cations and para- and ortho-methylated
compounds were observed as main reaction products after the
reaction temperature was increased.
From the in situ MAS NMR results obtained under continu-
ous-flow conditions, however, it is difficult to determine the
role of N,N,N-trimethylanilinium cations in the overall reaction
leading to the formation of NMA, NNDMA, and ring-alkylated
products on zeolite H-Y. Therefore, the in situ stopped-flow
(SF) technique was applied, allowing a detailed investigation
of the reaction pathway. After N,N,N-trimethylanilinium cations
were formed on zeolite H-Y under continuous-flow conditions,
the reactant flow was stopped and an in situ MAS NMR
investigation of the progressive reaction of these surface
compounds was performed at different reaction temperatures.
Decomposition of N,N,N-Trimethylanilinium Cations. As
mentioned above, the stopped-flow experiment could be achieved
by either directly observing11 the further transformation of
adsorbed compounds at reaction temperatures or adding a
purging period12 before that. Since it has the advantage of
selectively retaining the N,N,N-trimethylanilinium cations after
the purging period, we prefer the latter protocolsalthough the
results are of no difference by use of these two protocols in
this study. Details of the in situ SF MAS NMR experiment are
displayed in Figure 1. After the continuous injection of the
reactants at 473 K for 1 h with a molar 13CH3OH to aniline
ratio of 4:1 and a modified residence time of 13CH3OH of W/F
) 75 (g‚h)/mol (Figure 5a), the flow was stopped and the
catalyst was purged with dry nitrogen at ambient temperature
for 1 h. In the spectrum recorded subsequently at ambient
temperature, only the signal of N,N,N-trimethylanilinium cations
occurred at 58 ppm (Figure 5b). After increase of the temper-
ature to 498 K without starting the flow of reactants again, a
strong intensity increase of the signals at 48 and 39 ppm was
observed, accompanied by a decrease of the signal at 58 ppm
(Figure 5c), which indicates a decomposition of N,N,N-
trimethylanilinium cations to N,N-dimethylanilinium and N-
methylanilinium cations. At 523 K, additional signals at 16 and
21 ppm appeared due to ring-alkylated reaction products with
methyl groups in para and ortho positions, respectively (Figure
5d). The 13C-labeled methyl groups which are abstracted during
these decomposition processes may react with adsorbed aniline
molecules giving NMA and NNDMA and/or contribute to the
ring-alkylation leading to reaction products with methyl groups
in para and ortho positions.
Figure 5. In situ 13C MAS NMR spectra recorded during aniline
methylation on zeolite H-Y under continuous-flow conditions (W/F ) 75
(g‚h)/mol, 13CH3OH/aniline ) 4:1) at a reaction temperature of 473 K for
1.0 h (a), at 298 K after stopping the flow of reactants and purging the
catalyst with dry nitrogen (b), and, subsequently, at reaction temperatures
of 498 K (c) and 523 K (d) without purging the catalyst.
decomposition of tetramethylammonium cations at elevated
temperatures were obtained by 13C MAS NMR studies of
ammonia methylation on solid catalysts under batch condi-
tions.22,23 From an analysis of 13C MAS NMR data collected
during the hydrothermal synthesis of the AlPO4 material UiO-
12, Kongshaug et al. were led to propose that the tetramethyl-
ammonium cations used as a structure-directing agent may
decompose into dimethylammonium and small amounts of
methylammonium.26 To the best of our knowledge, however,
the decomposition behavior of quaternary anilinium cations in
acidic zeolites was never studied before under in situ conditions.
Mechanism of Aniline Methylation. Although the methyl-
ation of aniline on acidic catalysts has been investigated in a
number of previous studies, the reaction mechanism continues
to be a matter of discussion.14-16 In situ MAS NMR spectros-
copy performed under batch17 and continuous-flow conditions
now furnish direct insight into the intermediates on the surface
of the working catalyst during the methylation of aniline on
acidic zeolites. Especially, the chemical equilibria of the
anilinium cations presented in Scheme 1 deserve particular
attention.
Experimental studies established that the fastest step of the
methanol-to-olefin process (MTO) on acidic zeolites is the
conversion of methanol to DME.27 Generally, the starting
temperature of DME formation on acidic zeolites is as low as
ca. 423 K. Under continuous-flow conditions and at reaction
temperatures higher than 473 K, the equilibrium between
methanol and DME is greatly shifted to DME.2,12 The present
in situ CF MAS NMR results indicate that this conversion also
happens during aniline methylation on zeolite H-Y (Figure 2b).
Furthermore, the formation of N,N,N-trimethylanilinium cations
and other products is accompanied by a decrease of the surface
It is well established that the alkylation of ammonia with
methanol proceeds in a stepwise manner, the products being
methylamine, dimethylamine, trimethylamine, and the tetra-
methylammonium cation.25 On the other hand, hints for the
(26) Kongshaug, K. O.; Fjellvag, H.; Klewe, B.; Lillerud, K. P. Microporous
Mesoporous Mater. 2000, 39, 333-339.
(27) Stoecker, M. Microporous Mesoporous Mater. 1999, 29, 3-48.
(25) Linstromberg, W. W.; Baumgarten, H. E. Organic Chemistry. A Brief
Course, 5th ed.; pp 355-357.
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7552 J. AM. CHEM. SOC. VOL. 124, NO. 25, 2002

N,N,N-Trimethylanilinium Cations on Zeolite H−Y
A R T I C L E S
Scheme 1
concentration of DME (Figure 2c). The 13C MAS NMR results
obtained from methylation of aniline17 and ammonia22,23 by
methanol under batch conditions also showed that DME occurs
at low reaction temperatures and decreases or disappears at high
reaction temperatures. It is necessary to clarify whether DME
is involved in the methylation of aniline. Thus, the reaction of
DME itself with aniline on zeolite H-Y was studied by in situ
MAS NMR spectroscopy under batch conditions. The 13C MAS
NMR spectra obtained are shown in Figure 6. After heating of
zeolite H-Y externally loaded with DME and aniline at 453
K, the signal of N,N,N-trimethylanilinium cations occurred at
58 ppm along with the signal of N-methylanilinium cations at
39 ppm. Thus, it is very likely that DME can react with aniline,
although the reaction could have occurred via the conversion
of dimethyl ether to methanol through trace quantities of water
present in the catalyst. On the other hand, methoxy groups
prepared by adsorption and conversion of 13CH3I and 13CH3OH
under batch conditions can also react with aniline to NMA.17
In addition, in situ SF MAS NMR investigations12 of the
conversion of methanol to DME showed that methoxy species
(CH3OZ) formed from methanol can react with other methanol
molecules to yield DME. From these we conclude that methanol,
methoxy species, and DME are all possible alkylating agents
involved in the methylation of aniline under continuous-flow
conditions (Scheme 1, top).
protonation site of aniline and substituted anilines in the gas
phase,31 we believe that, according to their basic behavior in
solution, the protonation of aniline or substituted anilines on
zeolite H-Y occurs preferentially at the nitrogen atom rather
than at carbon atoms of the aromatic ring. In the methylation
of aniline, therefore, the N-alkylation preferentially occurs
instead of the C-alkylation32 because the N atom is more
nucleophilic. Protonation of ortho- or para-toluidines at the
nitrogen atom (proton affinities30 of 890.9 and 896.7 kJ/mol,
respectively) shows no influence on the 13C NMR shift of the
ortho- and para-methyl groups (21 and 16 ppm, respectively),
although it may cause similar low-field shifts of aromatic carbon
atoms as in the case of aniline.
The conversion of reactant mixtures with a molar methanol-
to-aniline ratio of 1:3 on zeolite H-Y under batch conditions
led to protonated NMA.17 On the contrary, with a molar
methanol-to-aniline ratio of more than 1:1 under flow conditions,
aniline methylation does not stop after the formation of
N-methylanilinium cations. Instead, N,N,N-trimethylanilinium
cations are formed at low reaction temperatures, together with
small amounts of N-methylanilinium and N,N-dimethylanilinium
cations. It should be noted that the weak 13C NMR signal of
N,N-dimethylanilinium cations overlaps with the signal of
adsorbed methanol molecules at 50 ppm (Figure 2b). However,
at reaction temperatures of 523 K and higher, N,N-dimethyl-
anilinium cations together with N-methylanilinium cations and
ring-methylated compounds occurred as the main products on
the working catalyst accompanied by a decrease of the N,N,N-
trimethylanilinium cations (Figure 2d). The in situ SF MAS
NMR results clearly indicate that at these reaction temperatures
N,N,N-trimethylanilinium cations decompose to N,N-dimethyl-
anilinium cations (Figure 5). All these observations can be
rationalized by invoking that the equilibrium between N-methyl-
N,N-dimethyl- and N,N,N-trimethylanilinium cations is estab-
lished, as shown in Scheme 1. At high methanol-to-aniline ratios
and low temperatures, the equilibrium shifts to N,N,N-trimethyl-
anilinium cations, while at low methanol-to-aniline ratios and
Compared with the 13C NMR shift data of NMA and
NNDMA in an inert solution (30.2 and 40.3 ppm, respectively)28
and of NMA on a basic zeolite CsOH/Cs,Na-Y (29 ppm),11
a
significant low-field shift of the methyl groups of NMA (39
ppm) and NNDMA (48 ppm) adsorbed on acidic zeolite H-Y
occurs. According to theoretical calculations performed by
Nicholas and Haw,29 the formation of a persistent cation on a
zeolite in its H+ form requires that the adsorptive has a proton
affinity of at least 875 kJ/mol. Thus, the above-mentioned low-
field shifts are most probably caused by a protonation of NMA
and NNDMA (proton affinity30 of 916.6 and 941.1 kJ/mol,
respectively) on the Brønsted acid sites of zeolite H-Y leading
to N-methylanilinium and N,N-dimethylanilinium cations. Ob-
serving the low-field 13C NMR shifts of aniline (proton affinity30
of 882.5 kJ/mol) adsorbed on zeolite H-Y, Ivanova et al.17 also
suggested a protonation of these molecules. Furthermore,
although there are still longstanding arguments on the preferred
(31) For recent papers, see: (a) Lee, S.; Cox, H.; Goddard, W. A., III;
Beauchamp, J. L. J. Am. Chem. Soc. 2000, 122, 9201-9205. (b) Russo,
N.; Toscano, M.; Grand, A.; Mineva, T. J. Phys. Chem. A 2000, 104, 4017-
4021. (c) Bagno, A.; Terrier, F. J. Phys. Chem. A 2001, 105, 6537-6542.
(d) Roy, R. K.; de Proft, F.; Geelings, P. J. Phys. Chem. A 1998, 102,
7035-7040.
(32) C-alkylated products were proposed to be formed by a further transformation
or isomerization of N-alkylated anilines. See: (a) Bandyopadhyay, R.;
Singh, P. S.; Rao, B. S. Appl. Catal., A 1997, 155, 27-39. (b) Pillai, R. B.
C. J. Indian. Chem. Soc. 1999, 76, 157-158. (c) Yang, S. M.; Pan, T. W.
J. Chin. Chem. Soc. 1995, 42, 935-941.
(28) Kalinowski, H. O.; Berger, S.; Braun, S. 13C NMR Spectroscpy; Georg
Thieme Verlag: Stuttgart, New York, 1984; pp 285-286.
(29) Nicholas, J. B.; Haw, J. F. J. Am. Chem. Soc. 1998, 120, 11804-11805.
(30) NIST Chemistry WebBook (http://webbook.nist.gov/chemistry).
9
J. AM. CHEM. SOC. VOL. 124, NO. 25, 2002 7553

A R T I C L E S
Wang et al.
Additionally, investigations of template decomposition upon
calcination of as-synthesized zeolites has recently received much
research interest.26,34,35 However, the decomposition mechanisms
of quaternary ammonium salts being commonly used as template
molecules are poorly understood. Thus, it can be expected that
in situ NMR studies of the decomposition of quaternary
ammonium cations in zeolites bear the potential of significantly
improving our understanding of these processes.
Conclusions
On the basis of the results of in situ 13C NMR investigations
performed in the present work, a mechanism of aniline methyl-
ation on acidic zeolite H-Y is proposed: In the first step of
the reaction, methanol is converted to DME and surface methoxy
groups, which are further involved in the methylation as
alkylating agents along with methanol. Alkylation starts at 473
K and leads to a consecutive and reversible formation of
N-methylanilinium, N,N-dimethylanilinium, and N,N,N-trimethyl-
anilinium cations attached to the zeolite surface. The products
of the N-alkylation of aniline, N-methylaniline and N,N-dimethyl-
aniline, are further formed via deprotonation of the corre-
sponding N-methylanilinium and N,N-dimethylanilinium cations.
We propose that the product distribution in the gas phase is
determined to a large extent by the chemical equilibrium
between the different methylanilinium cations, which is in
turn affected by the reaction conditions (temperature, molar
methanol-to-aniline ratio). C-alkylated products are formed
via transformation of methylanilinium cations at temperatures
higher than 523 K.
Figure 6. 13C MAS NMR spectra of zeolite H-Y recorded at 298 K after
heating at 373 K for 2.0 h (a) and at 453 K for 2.0 h (b). Zeolite H-Y was
externally loaded with a mixture of dimethyl ether and aniline in a molar
ratio of 2:1.
high temperatures N-methylanilinium and N,N-dimethylanilin-
ium cations are preferentially formed. The content of gaseous
products at the outlet of the flow reactor will further depend
not only on the equilibrium constants of formation and decom-
position of N,N,N-trimethylanilinium, N,N-dimethylanilinium,
and N-methylanilinium cations on the surface of the catalyst
(K1-K3) but also on the equilibrium constants of deproton-
ation and desorption of N-methylanilinium and N,N-dimethyl-
anilinium cations (K5, K6). Hence, the ratio of these constants
(K1:K2:K3:K5:K6) at different experimental conditions (temper-
ature, methanol to aniline molar ratio) determines the final
product distribution in the gas phase.
It should be mentioned that the reaction pathway including
the formation of quaternary cations and their subsequent
decomposition suggested in this work may, in some cases,
explain the appearance of unexpected products observed during
amine alkylation on acidic catalysts. For example, Pouilloux et
al. synthesized dimethylethylamine from ethylamine and metha-
nol on an acidic catalyst.33 At the reaction temperature of 503
K, an unexpected product, trimethylamine, was determined in
high yield by gas chromatographic analysis. This can be easily
explained by the formation and decomposition of trimethyl-
ethylammonium cations, (CH3)3N+CH2CH3, during the reaction.
Acknowledgment. Financial support by the Deutsche
Forschungsgemeinschaft, Volkswagen-Stiftung, Max-Buchner-
Forschungsstiftung, and Fonds der Chemischen Industrie is
gratefully acknowledged. I.I.I. thanks the Russian Foundation
of Fundamental Research for a research grant. We thank Dr.
W. Song (University of Southern California) and Dr. R. Gla¨ser
(University of Stuttgart) for their helpful disscussions.
JA012675N
(34) Kruk, M.; Jaroniec, M. J. Phys. Chem. B 1999, 103, 4590-4598.
(35) Kleitz, F.; Schmidt, W.; Schueth, F. Microporous Mesoporous Mater. 2001,
44/45, 95-109.
(33) Pouilloux, Y.; Doidy, V.; Hub, S.; Kervennal, J.; Barrault, J. Stud. Surf.
Sci. Catal. 1997, 108, 139-147.
9
7554 J. AM. CHEM. SOC. VOL. 124, NO. 25, 2002