In situ stopped-flow (SF) MAS NMR spectroscopy: a novel NMR
technique applied for the study of aniline methylation on a solid base
catalyst
a
a
b
a
a
Wei Wang, Michael Seiler, Irina I. Ivanova, Jens Weitkamp and Michael Hunger*
a
Institute of Chemical Technology, University of Stuttgart, D-75500 Stuttgart, Germany.
E-mail: michael.hunger@po.uni-stuttgart.de
Department of Chemistry, Moscow State University, Leninskie Gory, 119899 Moscow, Russia
b
Received (in Cambridge, UK) 10th May 2001, Accepted 15th June 2001
First published as an Advance Article on the web 5th July 2001
By the novel in situ stopped-flow MAS NMR technique
allowing the observation of adsorbates on a solid catalyst
after stopping the continuous reactant flow, N-methylenea-
niline was identified as an intermediate in the formation of
N-methylaniline by methylation of aniline with methanol on
a basic CsOH/Cs,Na-Y zeolite.
raised to 473, 498 and 523 K while the carrier gas (nitrogen)
loaded with the reactants was flowing. In all experiments, a
13
methanol ( C-enriched) flow according to a modified residence
2
1
time of W/F = 40 g h mol (mcat = 250 mg, n˙ me = 6.25 mmol
2
1
h
) was used. The molar ratio of the methanol–aniline
13
(natural C-abundance) mixture was 4+1. After recording
1
3
the C MAS NMR spectra under steady-state conditions at
reaction temperatures of 473, 498 and 523 K, the reactant flow
was stopped, and the further conversion of the adsorbate
compounds was observed, without purging the catalyst (see Fig.
1).
In the past decade, in situ MAS NMR spectroscopy has been
developed as a powerful tool for investigating heterogeneously
1
catalyzed reactions. Since 1995, a number of new in situ MAS
NMR techniques have been developed allowing the study of
reactions under continuous flow conditions.2 With these
techniques, a direct NMR investigation of the formation and
transformation of surface compounds under steady-state condi-
tions and a simultaneous gas chromatographic analysis of the
reaction products are possible. In the present communication we
report on the application of a new in situ stopped-flow (SF)
MAS NMR experiment which is suitable to determine inter-
mediates of heterogeneously catalyzed reactions. With this
method, an intermediate involved in aniline methylation on
basic zeolite CsOH/Cs,Na-Y under flow conditions was
determined for the first time.
Methanol was applied as methylating agent, and the conver-
sion of this reactant alone on zeolite CsOH/Cs,Na-Y was
investigated in the first experiments. Fig. 2(a)–(c), left, show the
in situ 13C MAS NMR spectra obtained at reaction temperatures
of 473–523 K under steady-state conditions. The signal at 49
ppm is due to adsorbed methanol molecules. With increasing
reaction temperature, a second signal appears at 166 ppm which
4
is caused by surface formate species. In an earlier work on
methylation of toluene on a basic zeolite CsOH/Cs,Na-X it was
shown that surface formate species occurring at 166 ppm are
consumed by the reaction which indicates that these species can
act as methylating agents.4 The C MAS NMR spectrum
recorded immediately after stopping the methanol flow at 523 K
shows a significant decrease of the methanol signal at 49 ppm
and only a weak decrease of surface formate species at 166 ppm
[Fig. 2(d), left]. This indicates that the surface formate species
are quite stable at 523 K.
c
13
The zeolite CsOH/Cs,Na-Y used as catalyst in the present
work had an nSi/nAl ratio of 2.6 and was prepared as described
3
elsewhere. After the sodium/caesium exchange (sodium ex-
change degree of 70%), zeolite Cs,Na-Y was impregnated with
an aqueous solution of caesium hydroxide such as to arrive at a
loading of 14 CsOH per unit cell. Subsequently, the material
was calcined for 12 h at 723 K. The NMR experiments were
performed on a Bruker MSL 400 spectrometer at a resonance
frequency of 100.4 MHz, with direct excitation (p/2 pulse), a
repetition time of 5 s and ca. 500 scans per spectrum. For the in
situ measurements, the equipment described in ref. 2(f) and a
modified 7 mm high-temperature Doty MAS NMR probe were
used. The protocol of the ‘stopped-flow’ experiment is shown in
Fig. 1. After filling the calcined catalyst into the MAS rotor and
transferring the rotor into the spectrometer, the temperature was
Fig. 2(a)–(c), right, show the 13C MAS NMR spectra
recorded during conversion of the methanol–aniline mixture on
zeolite CsOH/Cs,Na-Y at reaction temperatures of 473–523 K.
While the spectrum obtained at 473 K consists only of a single
signal due to methanol molecules at 49 ppm, in the spectrum
obtained at 498 K additional signals occur at 29 and 157 ppm.
The signal at 29 ppm is due to the reaction product N-
5
methylaniline. The signal at 157 ppm can be assigned to N-
methyleneaniline, based on work by Kamachi and coworkers
13
who observed a signal at ca. 155 ppm in the C NMR spectrum
6
8
of N-methyleneaniline in THF-d . Further increase of the
reaction temperature to 523 K leads to the formation of surface
formate species at 166 ppm and a strong increase of the signal
at 29 ppm due to the reaction product N-methylaniline.
To find out whether the N-methyleneaniline species at 157
ppm is an intermediate in the formation of N-methylaniline on
zeolite CsOH/Cs,Na-Y, a stopped-flow experiment was per-
formed: the reactant flow was suddenly stopped, and a spectrum
was recorded while keeping the temperature at 523 K. In the
spectrum obtained [Fig. 2(d), right], the signal at 166 ppm
remained constant. On the other hand, the signals of methanol at
4
9 ppm and of N-methyleneaniline at 157 ppm disappeared
completely, while the signal of the reaction product N-
methylaniline at 29 ppm gained significantly in intensity.
Hence, not only methanol, but also the N-methyleneaniline
species are consumed by the formation of N-methylaniline. This
finding indicates that the N-methyleneaniline species occurring
Fig. 1 Protocol of the in situ SF MAS NMR experiment consisting of
periods (a) to (c) for the study of heterogeneously catalyzed reactions under
steady-state conditions at different temperatures and a period (d), in which
the consecutive conversion of previously formed compounds can be
investigated.
1362
Chem. Commun., 2001, 1362–1363
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b104115k