Parahydrogen-induced polarization (PHIP) in heterogeneous hydrogenation over bulk metals and metal oxides

Article abstract of DOI:10.1039/c3cc44939d

The results show that, in general, observation of PHIP effects on metals does not require the presence of a support. In addition, certain metal oxides can produce PHIP effects in the absence of a supported metal. The new promising catalysts for producing PHIP heterogeneously have been identified. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc44939d

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Parahydrogen-Induced Polarization (PHIP) in heterogeneous
hydrogenations over bulk metals and metal oxides
Kirill V. Kovtunov,*a,b Danila A. Barskiy, a,b Oleg G. Salnikov, a,b Alexander K. Khudorozhkov,c Valery I.
Bukhtiyarov,c Igor P. Prosvirinc and Igor V. Koptyug a,b
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
observed in heterogeneous hydrogenation reactions catalyzed by
50 supported metal catalysts, both in liquid and gaseous phases.8,9 In
addition, it was established that not only the nature of the
supported metal but also the type of support can have a dramatic
influence on the observed PHIP effects.10-12 For hydrogenation of
propyne, propylene and 1,3-butadiene with parahydrogen over
55 catalysts comprising noble metals supported on oxides
(M/MexOy; M = Pt, Pd, Rh; Me = Al, Ti, Si, Zr), it was shown
that the intensities of PHIP signals depend on the support used,
with the largest polarization detected for metals supported on
TiO212,13. In contrast, bulk metal surfaces are generally not
60 expected to produce PHIP effects because the accepted
mechanism of heterogeneous hydrogenation involves the
dissociative chemisorption of H2 on the metal surface. This
should randomize the H atoms on the surface and destroy
quantum correlations between them. The absence of the pairwise
65 hydrogen addition over bulk metal surfaces was apparently
confirmed in previous studies where hydrogenation of propylene
with parahydrogen over Pt metal film produced no PHIP
effects.14,15 At the same time, one study reports the observation of
PHIP effects upon reversible chemisorption of parahydrogen on
70 the surface of ZnO,16 providing a hint that bulk oxides may be
able to produce PHIP in heterogeneous hydrogenations.
These and other results clearly show the important role of both
the supported metal and the oxide support in the reaction
mechanisms that lead to PHIP observation. At the same time, the
75 exact nature of the active sites that can implement pairwise
addition of H2 to a substrate on supported metal catalysts and the
mechanisms of the support influence on the PHIP effects are not
fully established. In this study, we address these important issues
and report for the first time the PHIP effects produced using bulk
80 metals and metal oxides as heterogeneous hydrogenation
catalysts.
The results show that, in general, observation of PHIP effects
on metals does not require the presence of a support. In
addition, certain metal oxides can produce PHIP effects in the
10 absence of a supported metal. The new promising catalysts
for producing PHIP heterogeneously have been identified.
Catalysis by metals is an important part of the catalysis field.
Taking into account that heterogeneous catalysis is an interface
phenomenon, metals are often dispersed over suitable supports
15 and used as metal nanoparticles deposited on various oxides.1,2
Therefore, real heterogeneous catalysts are supported metal
nanoparticles with sizes below 10 nm.3 Amphoteric or transition
metal oxides are often used as supports in heterogeneous
catalysis. They increase metal dispersion and prevent
20 agglomeration of metal particles during the reaction.4
Noble metals such as palladium, platinum, nickel and rhodium
are very efficient in heterogeneous catalytic hydrogenations of
unsaturated compounds.1 It should be noted, however, that not
only bulk or supported metals but also metal oxides can
25 heterogeneously catalyze hydrogenation reactions. In particular,
utilization of alkaline-earth metal oxides such as MgO, BaO, SrO,
BeO and CaO may result in selective hydrogenation of
unsaturated hydrocarbons.4 Another example is the
heterogeneous hydrogenation of ethylene over chromia and zinc
30 oxide which proceeds through an irreversible two-step hydrogen
addition with H-O-Me-H as the probable transition state.5
Importantly, the half-hydrogenated intermediate cannot migrate
from one -O-Me- site to another and, therefore, hydrogen may
preserve its molecular identity if H migration over the oxide
35 surface is not too fast.
Due to the complex structure of the real catalysts and
intermediates that are formed during the hydrogenation reaction,
the development of new techniques and methods is necessary for
the detection of products and intermediates that are present at low
40 concentrations. One of the useful techniques for such
investigations is based on a significant NMR signal enhancement
provided by parahydrogen.6 Parahydrogen-induced polarization
(PHIP) may be successfully utilized for investigating the
mechanisms of homogeneous reactions that involve metal
45 complexes based on rhodium, iridium, ruthenium, etc., as
catalysts, and to identify reaction intermediates even at low
concentrations.7
In the previous PHIP studies with supported catalysts, Al2O3,
TiO2, SiO2 and ZrO2 were typically used as supports for noble
metals. These oxides have a negligible activity in hydrogenation.
85 Indeed, no polarization could be observed when they were used
as catalysts in hydrogenations of unsaturated compounds with
parahydrogen. Next, CaO, Cr2O3, ZrO2 and CeO2 were used as
catalysts for the heterogeneous hydrogenation of propylene and
1,3-butadiene. Electronic Supplementary Information (ESI)
90 provides the details on catalysts preparation and activation and
the experimental setup. For the reactions performed inside the
It has been shown recently that PHIP may be also successfully
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45 Fig. 2. 1H NMR spectra detected upon hydrogenation of 1-butyne with
parahydrogen over several Pt- and Pd-containing catalysts at 130° C. All
spectra were recorded under identical conditions. Note that the spectrum
for Pt black is scaled down relative to other spectra.
1
Fig. 1. H NMR spectrum detected upon hydrogenation of 1,3-butadiene
with parahydrogen over CaO at 130 °C in the NMR probe (the
PASADENA experiment).
results in the predominant formation of CH2D-CH2D and CH3-
50 CH3 as compared to CH2D-CH3.5 Our observation of PHIP
effects over bulk oxides confirms the formation of the R-CHHa-
CH2Hb product molecule where Ha and Hb hydrogens come from
the same parahydrogen molecule (Ha-Hb), and thus establishes the
importance of the pairwise hydrogen addition to the substrate in
55 the reaction mechanism. These observations open up new
possibilities for developing stable and non-toxic materials for
PHIP production.
5
NMR probe (the PASADENA17 experiment), the maximum
temperature was limited to 130 °C. At this temperature, activity
of CaO in the hydrogenation of propylene was low. Nevertheless,
low-intensity PHIP effects could be observed. It was argued that
stronger adsorption of a substrate with a longer hydrocarbon
10 chain and more unsaturated C=C bonds may result in more
efficient hydrogenation and pairwise hydrogen addition. Indeed,
hydrogenation of 1,3-butadiene with parahydrogen over CaO
produced very pronounced PHIP effects for all reaction products
(1- and 2-butenes and butane, Fig. 1).
As mentioned earlier, bulk metals are not expected to perform
the pairwise hydrogen addition and therefore should not be
60 suitable for PHIP observation. Contrary to these expectations, we
demonstrate that utilization of parahydrogen in the hydrogenation
of propylene, 1,3-butadiene, 1-butyne and acrolein catalyzed by
Pt black does produce PHIP effects of the reaction products.
When 1-butyne was hydrogenated over Pt black, 1-butene, 2-
65 butene and butane were formed. Importantly, all three protons of
the vinyl fragment H2C=CH– of 1-butene are polarized (Fig. 2).
2-Butene is formed in small amounts as confirmed by the low
intensity of its CH group NMR signal. Similar spectra are
observed upon 1-butyne hydrogenation over PtO2 and Pt(OH)2,
70 although the intensity of polarized lines is different (Fig. 2). In
contrast, the Pd black catalyst produced no polarization for any of
the substrates mentioned above despite the relatively high
conversion levels. The different behavior of Pd compared to Pt is
likely due to the involvement of the hydride phase of Pd
75 (PdHx)21,22 in the reaction which destroys spin correlation of the
H atoms coming from parahydrogen. Similar to Pd, the Pd(OH)2
catalyst is not able to produce PHIP for any of the substrates. In
contrast, for PdO it is possible to observe polarization (Fig. 2).
These observations verify the fact that various metal oxides are
80 able to perform pairwise hydrogen addition and to produce PHIP
effects. To further confirm this, X-ray photoelectron spectroscopy
(XPS) was used to characterize bulk Pd, Pt, PdO and PtO2
samples. XPS data show that before the reaction the metals are
present only in one oxidation state in each sample (Pd(0), Pt(0),
85 Pd(II) and Pt(IV), respectively; Fig. S4 in the ESI). Therefore,
bulk metals are definitely in the zero oxidation state during the
hydrogenation, whereas it is possible that PdO and PtO2 may be
partially reduced in the hydrogen atmosphere. In the case of
15
Conducting the reaction outside the NMR magnet with the
subsequent sample transfer to the NMR probe for spectrum
acquisition (the ALTADENA18 experiment) provides much more
flexibility in the experimental conditions. In particular, much
higher temperatures and larger catalyst volumes can be used.
20 Hydrogenation of 1,3-butadiene with parahydrogen over CaO in
the ALTADENA experiment at 400 °C demonstrated significant
PHIP effects (see Fig. S1 in the ESI). Analogously, propylene
hydrogenation over CaO under ALTADENA reaction conditions
showed that increasing the reaction temperature up to 500 °C
25 resulted in the increase of propylene conversion to propane and
the polarization of the CH2 and CH3 groups of propane (Fig. S2
in the ESI).
Another oxide, Cr2O3, produced PHIP effects only at high
temperatures, with activity depending on the preparation
30 procedure (Fig. S3 in the ESI). For ZrO2, some small polarization
was observed in the 1,3-butadiene hydrogenation at the high gas
flow rates under ALTADENA conditions. PHIP effects were also
observed for propylene hydrogenation over CeO2 at 600°С at
moderate flow rates. Further details of the experiments with
35 Cr2O3, ZrO2 and CeO2 are given in ESI.
The results presented above prove that PHIP effects may be
observed in heterogeneous hydrogenations of unsaturated
hydrocarbons over bulk metal oxides. One of the plausible
explanations of this fact can be the reduced mobility of surface H
40 atoms which may be directly related to the strength of adsorbate-
support interaction. It is well known that a number of oxides may
act as hydrogenation catalysts.19,20 Moreover, the heterogeneous
hydrogenation of ethylene with H2+D2 over Cr2O3 and ZnO
2
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palladium, PHIP effects were observed only on the oxide phase
entirely necessary to have both the metal and the support for the
but not on the metal. It means that Pd(II), and not Pd(0), is 60 observation of PHIP effects and, therefore, for the pairwise
required for PHIP effect observation, at least in the case of bulk
catalysts. For platinum the situation is different, because
polarized products were detected in the hydrogenations over bulk
metal as well as oxide and hydroxide. Therefore, the pairwise
hydrogen addition to take place. These findings may help
establish the nature of the catalytic centers that are responsible for
PHIP effects, and can lead to the development of the new types of
heterogeneous catalysts with high levels of pairwise hydrogen
5
hydrogen addition can occur on both oxidized and non-oxidized 65 addition and, therefore, large enhancements of the NMR signals,
platinum.
When 1,3-butadiene was hydrogenated on bulk PtO2, quite
by properly combining appropriate oxides and metals in the
catalyst structure.
10 intense polarized NMR signals of 1-butene, 2-butene and butane
were observed despite the low conversion. This situation is
This work was supported by the RAS (5.1.1), RFBR (11-03-
00248-а, 12-03-00403-a), SB RAS (57, 60, 61, 122), the program
similar to that for CaO (Fig. 1). It is interesting that all NMR 70 of support of leading scientific schools (NSh-2429.2012.3), the
signals of all products are polarized including the signals of all
protons (CH and CH2 groups) of the vinyl fragment of 1-butene
15 and the protons of the CH group of 2-butene. This may be
explained assuming that 2-butene is formed by a reversible
isomerization of 1-butene. Therefore, the reaction path leading to
the polarized butenes should start with the pairwise addition of
the two H atoms from the same H2 molecule to the C=C bond of
20 1,3-butadiene (Scheme S1 in the ESI). We note that 1,4-hydrogen
addition to 1,3-butadiene results in the formation of 2-butene
directly, and in principle this reaction route cannot be ruled out.
However, the intensity and shape of the polarized lines in this
case should be different. In the case of bulk Pt and Pt(OH)2
25 catalysts, all products (1-butene, 2-butene and butane) are
polarized, but with a much lower intensity (Fig. S5 in the ESI).
Therefore, contribution of the non-pairwise route(s) of hydrogen
addition in the case of Pt and Pt(OH)2 catalysts is higher than in
the case of PtO2. In the case of 1,3-butadiene hydrogenation over
30 Pd-containing catalysts, polarization was observed only over
PdO, so the hydrogen addition over bulk Pd and Pd(OH)2
catalysts is entirely non-pairwise.
program of the Russian Government to support leading scientists
(11.G34.31.0045) and the Council on Grants of the President of
the Russian Federation (МК-4391.2013.3).
Notes and references
75 a International Tomography Center, 3A Institutskaya St., Novosibirsk
630090, Russia. Fax: +7 383 333 1399; Tel: +7 383 330 7926; E-mail:
kovtunov@tomo.nsc.ru
b Novosibirsk State University, 2 Pirogova St., Novosibirsk, 630090,
Russia
80 c Boreskov Institute of Catalysis SB RAS, 5 Acad. Lavrentiev Pr.,
Novosibirsk 630090, Russia
† Electronic Supplementary Information (ESI) available: catalysts
preparation, activation, NMR experiments and data evaluation. See
DOI: 10.1039/b000000x/
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40 hydrogenation of 1,3-butadiene and 1-butyne over Pt black and in
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50 but excludes pairwise hydrogen addition.
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confirmation that the mechanisms of hydrogenation over bulk
metals as well as metal oxides sustains the transfer of both
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55 product molecule. The similarities between metals and oxides
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