Separation of Anti-Phase Signals Due to Parahydrogen Induced Polarization via 2D Nutation NMR Spectroscopy

Article abstract of DOI:10.1002/cphc.201601227

The present work introduces a novel method for the selective detection of ¹H NMR anti-phase signals caused by the pairwise incorporation of parahydrogen into olefins on noble-metal-containing catalysts. Via a two-dimensional (2D) nutation NMR experiment, the anti-phase signals of hyperpolarized ¹H nuclei are separated due to their double nutation frequency compared to that of thermally polarized ¹H nuclei. For demonstrating this approach, parahydrogen induced polarization (PHIP) was achieved via the hydrogenation of propene with parahydrogen on platinum-containing silica and investigated by in situ ¹H MAS NMR spectroscopy under continuous-flow conditions, that is, the hydrogenation reaction was performed inside the magnet of the NMR spectrometer. The 2D nutation NMR experiment described in the present work is useful for the separation of overlapping anti-phase and in-phase signals due to hyperpolarized and thermally polarized ¹H nuclei, respectively, which is important for research in the field of heterogeneous catalysis.

Full text of DOI:10.1002/cphc.201601227

DOI: 10.1002/cphc.201601227
Communications
Separation of Anti-Phase Signals Due to Parahydrogen
Induced Polarization via 2D Nutation NMR Spectroscopy
Utz Obenaus,[a] Gerhard Althoff-Ospelt,[b] Swen Lang,[a] Robin Himmelmann,[a] and
Michael Hunger*[a]
The present work introduces a novel method for the selective
under study to a reservoir of nuclei with much higher polariza-
tion.[6] A suitable route is the pairwise incorporation of the two
hydrogen atoms of parahydrogen molecules into olefins lead-
ing to reaction products with parahydrogen induced polariza-
tion.[7–9] In this case, a large non-equilibrium spin polarization,
that is, a hyperpolarization, may occur. Normal hydrogen gas
(assigned n-H2 in this work) has a para (nuclear spin I=0) to
ortho (nuclear spin I=1) ratio of 1:3, which can be converted
to a para to ortho ratio of 1:1 (assigned p-H2 in this work) by
contacting n-H2 with activated charcoal or iron oxide at
77 K.[9,[10] The PASADENA (Parahydrogen And Synthesis Allow
Dramatically Enhanced Nuclear Alignment) protocol applied in
the present work is based on the hydrogenation of olefins
with p-H2 inside the magnet of the NMR spectrometer. In the
1H NMR spectra of the reactants, this procedure is accompa-
nied by an appearance of characteristic anti-phase signals,
which are due to a pairwise incorporation of the hydrogen
atoms of p-H2 into the olefin molecules.[9–14] In comparison
1
detection of H NMR anti-phase signals caused by the pairwise
incorporation of parahydrogen into olefins on noble-metal-
containing catalysts. Via a two-dimensional (2D) nutation NMR
1
experiment, the anti-phase signals of hyperpolarized H nuclei
are separated due to their double nutation frequency com-
1
pared to that of thermally polarized H nuclei. For demonstrat-
ing this approach, parahydrogen induced polarization (PHIP)
was achieved via the hydrogenation of propene with parahy-
drogen on platinum-containing silica and investigated by in
1
situ H MAS NMR spectroscopy under continuous-flow condi-
tions, that is, the hydrogenation reaction was performed inside
the magnet of the NMR spectrometer. The 2D nutation NMR
experiment described in the present work is useful for the sep-
aration of overlapping anti-phase and in-phase signals due to
1
hyperpolarized and thermally polarized H nuclei, respectively,
which is important for research in the field of heterogeneous
catalysis.
1
with the H NMR signals caused by the hydrogenation of the
same olefin molecules with n-H2, the above-mentioned anti-
phase signals can show an intensity enhancement by up to
three orders of magnitude due to the PHIP effect.[9]
The heterogeneously catalyzed hydrogenation of hydrocarbons
is an important reaction in petrochemistry and refining. Exam-
ples are the selective conversion of double- and triple-bond-
containing organic compounds with and without functional
groups into desired products, the purification of feedstocks for
polymerization reactions from polyenes, which are poisoning
the polymerization catalysts, and the elimination of alkynes in
gas streams of alkenes.[1–4] In the past decades, in situ solid-
state NMR spectroscopy under flow conditions has demon-
strated an increasing potential for investigating the mecha-
nisms of heterogeneously catalyzed reactions.[5] Broad applica-
tion of this method, however, requires an enhancement of the
sensitivity for the detection of intermediates and active surface
sites. An interesting strategy for enhancing the sensitivity of
NMR spectroscopy for the investigation of heterogeneously
catalyzed reaction systems is the coupling of the nuclear spins
In the past decade, an increasing number of groups utilized
heterogeneously catalyzed hydrogenation reactions for the in-
vestigation of PHIP effects.[15–19] Some studies had the aim to
form hyperpolarized propane via the heterogeneously cata-
lyzed hydrogenation of propene for applications in the field of
magnetic resonance imaging (MRI).[20–22] Furthermore, the
progress in the formation of PHIP via heterogeneously cata-
lyzed hydrogenation reactions allowed investigations of reac-
tion mechanisms.[23–27] In all these studies, the NMR detection
of the hyperpolarized reaction products was performed by in-
vestigating fluid reactant phases. NMR studies of hyperpolar-
ized reaction products interacting with the pore walls or the
particle surfaces of solid catalysts, on the other hand, require
the application of high-resolution solid-state NMR techniques,
such as magic angle spinning (MAS), as done in the present
work.
[a] U. Obenaus, S. Lang, R. Himmelmann, Prof. Dr. M. Hunger
Institute of Chemical Technology
Especially for porous solids, rapid relaxation of the PHIP of
nuclei in the reaction products (in this work called hyperpolar-
ized nuclei) on the particle surface and inside the pores may
occur. This relaxation can lead to intensities of the anti-phase
University of Stuttgart
70550 Stuttgart (Germany)
E-mail: michael.hunger@itc.uni-stuttgart.de
[b] Dr. G. Althoff-Ospelt
1
Solid-State NMR Application
Bruker BioSpin GmbH
Silberstreifen 4, 76287 Rheinstetten (Germany)
NMR signals of hyperpolarized H nuclei comparable to those
1
of the in-phase NMR signals of thermally polarized H nuclei. In
this case, it would be useful to have an experimental tech-
nique available, which allows the separation of anti-phase sig-
nals and in-phase signals. The present work demonstrates that
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
cphc.201601227.
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such a signal separation is possible along the second dimen-
sion of 2D nutation NMR spectra. This approach is based on
the specific properties of hyperpolarized nuclei giving maxi-
mum signal intensities after excitation with a p/4 pulse in con-
trast to thermally polarized nuclei requiring excitation by a p/2
pulse for reaching maximum signal intensities.[28,29] Corre-
spondingly, the nutation frequency n1 of hyperpolarized nuclei
in the radio frequency (nrf) field of the excitation pulse is factor
two higher compared to that of thermally polarized nuclei. A
similar effect is known for quadrupolar nuclei, such as 23Na
with a nuclear spin of I=3/2, which may occur at sites with
significantly different quadrupole frequencies nq.[30,31] In this
case, 2D nutation NMR experiments were utilized for separat-
ing the solid-state NMR signals of quadrupolar nuclei with
weak (nq <nrf) and strong (nq @nrf) quadrupolar interactions
along the second dimension (n1-axis) of 2D nutation NMR spec-
tra at n1 =1nrf and n1 =2nrf, respectively.[32–34]
perimental Section. For the hydrogenation of acrylonitrile on
this catalyst according to the procedure described in litera-
ture,[37] a reaction rate of (7.6Æ1.1)ꢁ10À4 mmolsÀ1 was ob-
tained. Under the same conditions, reaction rates of (2.3Æ
0.4)ꢁ10À4 mmolsÀ1 and (1.4Æ0.2)ꢁ10À3 mmolsÀ1 were deter-
mined for zeolites 0.8Pt/H,Na-Y and 4.5Pt/H,Na-Y, respective-
ly,[37] which indicate the high hydrogenation activity of the
0.9Pt/silica catalyst utilized in this study.
In the present work, the hydrogenation of propene with p-
H2 on 0.9Pt/silica was performed inside a modified 4 mm MAS
NMR rotor spinning with 4 kHz (Figures S1 and S2), with con-
tinuous propene and p-H2 flow rates of 40 and 30 mLminÀ1
,
respectively, and at the reaction temperature of 373 K. For
experimental details, see Supporting Information. As control
experiments, in situ 1H MAS NMR investigations with a fixed
single pulse p/4 excitation, that is, one-dimensional (1D) in situ
MAS NMR measurements were performed during the hydroge-
nation of propene with n-H2 and p-H2 leading to the spectra in
Figure 2a and Figure 2b, respectively. The spectrum in Fig-
Similarly to the 2D nutation MAS NMR investigations of
quadrupolar nuclei, the present 2D nutation NMR experiments
with hyperpolarized 1H nuclei were performed by incrementing
the pulse length described by the time domain t1, while the in-
duction decay is recorded as a function of the time domain t2
(Figure 1, left). The two-dimensional Fourier-transformation (2D
FT) of the data as a function of t1 and t2 gives a 2D nutation
NMR spectrum with the chemical shifts of the observed signals
along the F2-axis and their nutation frequencies n1 along the
F1-axis (Figure 1, right).
1
ure 2a consists exclusively of signals of thermally polarized H
nuclei due to propene at 1.3Æ0.2 ppm, 4.8Æ0.2 ppm, and
5.6Æ0.2 ppm and of the reaction product propane at 0.6Æ
0.2 ppm and 1.0Æ0.2 ppm. Differences between the gas phase
chemical shifts in the present work and chemical shifts in the
literature[38] are due to the different surroundings of the mole-
cules under study. The additionally observed broad shoulder at
1.6Æ0.2 ppm is caused by surface OH groups of the silica sup-
port.[34] In the spectrum shown in Figure 2b, the signals of
thermally polarized 1H nuclei are overlapped by typical anti-
phase signals due to the pairwise incorporation of p-H2 into
propene. In Figure 2c, the difference spectrum consisting ex-
clusively of the anti-phase signals is shown. The chemical shift
Figure 1. Scheme of the 2D nutation NMR experiment and the nutation
spectrum obtained upon 2D Fourier transformation (2D FT) for the time
domains t1 (incremented pulse length) and t2 (induction decays).
Very recently, first in situ continuous-flow MAS NMR spectro-
scopic investigations of the formation of PHIP on noble metal-
containing solids were performed with the aim to develop
novel experimental approaches for mechanistic studies in the
field of heterogeneous catalysis.[35,36] In these studies, the hy-
drogenation of propene with p-H2 in the gas phase was carried
out on solid catalysts filled into a MAS NMR rotor utilized as
a spinning micro-reactor. Based on these experiments, the
present 2D nutation NMR experiments aiming at the separa-
tion of the signals of hyperpolarized and thermally polarized
1H nuclei were performed during in situ hydrogenation of pro-
pene with p-H2 on a platinum-containing silica catalyst (0.9Pt/
silica) inside a spinning 4 mm MAS NMR rotor. The 0.9Pt/silica
catalyst was prepared and pretreated as described in the Ex-
Figure 2. In situ 1D 1H MAS NMR spectra recorded during the hydrogenation
of propene with n-H2 (top) and p-H2 (middle) on 0.9Pt/silica under continu-
ous-flow conditions at 373 K. The bottom spectrum is the difference of the
spectra recorded during hydrogenation with n-H2 and p-H2.
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values of these signals agree with those of propane (0.6Æ
0.2 ppm and 1.0Æ0.2 ppm).
1
The in situ 2D H nutation MAS NMR spectra shown in Fig-
ures 3 and 4 were recorded with a pulse power of 50 W corre-
1
sponding a nutation frequency of thermally polarized H nuclei
of 55 kHz, pulse lengths between 0.1 and 51.3 ms, that is, with
512 increments of 0.1 ms, eight scans per data set, and with
a repetition time of D1=0.1 s. Measurements with repetition
times of up to D1=20 s evidenced that no saturation occurs
for 1H NMR signals of the continuously flowing (30 and
40 mLminÀ1) reactants recorded with D1=0.1 s.
1
The in situ 2D H nutation MAS NMR spectrum in Figure 3
was recorded during hydrogenation of propene with n-H2 at
373 K. The slice obtained at n1 =1nrf =55 kHz corresponds to
the spectrum that consists of the signals of thermally polarized
1H nuclei (Figure 3, top). Correspondingly, in-phase signals of
propene at 1.3Æ0.2, 4.8Æ0.2, and 5.6Æ0.2 ppm and of the re-
action product propane at 0.6Æ0.2 and 1.0Æ0.2 ppm occur.
These shift values agree with those observed in the in situ 1D
1H MAS NMR spectrum in Figure 2a. No signals occur for the
slice obtained at n1 =2nrf =110 kHz in Figure 3, bottom.
Hydrogenation of propene with p-H2 on the 0.9Pt/silica cata-
1
lyst led to the in situ 2D H nutation MAS NMR spectrum in
Figure 4. Again, the slice at n1 =1nrf =55 kHz corresponds to
the spectrum that consists of the in-phase signals of thermally
polarized 1H nuclei (Figure 4, top). In comparison with the
spectrum in Figure 3, top, however, the signals of the reaction
product propane at 0.6Æ0.2 ppm and 1.0Æ0.2 ppm are much
weaker. Instead, the slice of the nutation spectrum at n1 =
2nrf =110 kHz (Figure 4, bottom) shows pure anti-phase signals
Figure 4. In situ 2D 1H nutation MAS NMR spectrum (middle) recorded
during hydrogenation of propene with p-H2 on 0.9Pt/silica at 373 K and
slices of this spectrum at the nutation frequencies of 1nrf (top) and 2nrf
(bottom).
of hyperpolarized nuclei at 0.6Æ0.2 ppm and 1.0Æ0.2 ppm,
caused by a pairwise incorporation of p-H2 into propene.
Hence, by sampling the nutation frequency of thermally polar-
1
ized and hyperpolarized H nuclei during the radio frequency
pulse via a 2D NMR experiment, the in-phase and anti-phase
signals of the above-mentioned nuclei are well-separated
along the F1 dimension. In future, this new experimental ap-
proach allows a more detailed investigation of the formation
and relaxation of PHIP of the product molecules of hydrogena-
tion reactions on solid catalysts, especially for the case of over-
lapping signals of thermally polarized and hyperpolarized
nuclei.
Summarizing, the present work demonstrates the possibili-
ties and advantages of 2D nutation NMR spectroscopy for the
1
selective detection of anti-phase signals of hyperpolarized H
nuclei caused by the formation of parahydrogen induced po-
larization (PHIP) via the PASADENA protocol. This experimental
1
approach, in combination with in situ H MAS NMR spectrosco-
py under continuous-flow conditions, opens novel possibilities
for mechanistic studies of heterogeneously catalyzed hydroge-
nation reactions via PHIP.
Experimental Section
The commercially available silica Aerosil 300 (Degussa AG, Hanau,
Germany) was used as delivered. The platinum-containing silica
was obtained by stirring 5 g of the silica material in 100 mL of
demineralized water at 313 K for 2 h and subsequent addition of
Figure 3. In situ 2D 1H nutation MAS NMR spectrum (middle) recorded
during hydrogenation of propene with n-H2 on 0.9Pt/silica at 373 K and
slices of this spectrum at the nutation frequencies of 1nrf (top) and 2nrf
(bottom).
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Acknowledgements
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Keywords: heterogeneous catalysis · hydrogenation · NMR
spectroscopy · parahydrogen induced polarization · propene
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Manuscript received: November 9, 2016
Revised: December 15, 2016
Final Article published: && &&, 0000
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COMMUNICATIONS
Hydrogenation: Two-dimensional nuta-
tion NMR experiments are demonstrat-
ed to be suitable for separating anti-
phase 1H NMR signals caused by parahy-
drogen induced polarization (PHIP). The
pairwise incorporation of parahydrogen
into propene is investigated on plati-
U. Obenaus, G. Althoff-Ospelt, S. Lang,
R. Himmelmann, M. Hunger*
&& – &&
Separation of Anti-Phase Signals Due
to Parahydrogen Induced Polarization
via 2D Nutation NMR Spectroscopy
1
num-containing silica using in situ H
MAS NMR spectroscopy under continu-
ous-flow conditions.
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These are not the final page numbers! ÞÞ