Jahresbericht 2007 - Leibniz-Institut für Katalyse

Magnetische Resonanz- und Röntgenmethoden Magnetische Resonanz- und Röntgenmethoden
Themenleiterin:
Priv.-Doz. Dr. Angelika
Brückner
Tel.: (030) 6392 – 4301
Fax: (030) 6392 – 4454
angelika.brueckner
@catalysis.de
operando-Druck-EPR
Kooperationspartner:
Dr. D- Maschmeyer,
Oxeno Olefinchemie
GmbH
Operando-high pressure-EPR:
A new tool for monitoring catalytic
reactions under industry-like
conditions demonstrated for olefin
oligomerization over Ni catalysts
Introduction and Objectives
Supported NiO catalysts are used since many years for the
heterogeneous catalytic oligomerization of low olefins. In the case of
butenes, there are two possible reaction pathways: 1) isomerization
to undesired strongly branched products over acidic support sites
dominating in the initial reaction period and 2) oligomerization
to linear C -C olefins over Ni sites becoming prevalent during
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equilibration of the catalysts [1]. Despite extensive research in the
field, the behaviour of Ni sites during equilibration and their nature
in the active state is still controversially discussed, considering both
single Ni + or Ni2+ as well as Ni0 species as being active. From ex-situ
EPR and FMR studies, we obtained hints that the equilibration process
might be governed by a dynamic segregation and redissolution of
Ni0 particles [1]. However, for deriving reliable structure-function
relationships, spectroscopic studies under true reaction conditions
are inevitable. In this work, we have adapted EPR spectroscopy for
monitoring the behaviour of Ni sites in different catalysts during
oligomerization of butenes (Raffinat III). To the best of our knowledge,
this is currently the only example of operando-high pressure EPR in
catalytic liquid/solid systems.
Results and Discussion
A thick-wall tubular quartz reactor has been directly implemented
in the cavity of a Bruker ELEXSYS 500-10/12 cw-EPR spectrometer
and connected to a liquiflow meter and a pressure controller for
maintaining a pressure of 20 bar and to a GC for online product
analysis (Fig. 1). Two different types of catalysts have been monitored
under Raffinat III flow at 80 °C: an industrial NiO/SiO -Al O catalyst
2 2 3
(NiO) and various 0.3 - 6 % Ni/SiO -Al O prepared by impregnation
2 2 3
with an organic Ni complex (0.3Ni – 6Ni). In NiO, the dissolution of
initially formed ferromagnetic Ni0 clusters was detected just with the
onset of catalytic activity. It was supposed that this process is related
to oxidative addition of Brønsted sites, Ni0 + H + → Ni + –H, but the
related Ni + species could not be detected due to superposition with
the intense Ni0 line. However, the typical rhombic EPR signal of Ni +
has been found together with ferromag-netic Ni0 in catalysts 0.3Ni –
6Ni. In 6Ni, formation of the latter was completed after 90 min time
on stream while the catalyst started to become active only after 300
min. This shows without any doubt, that ferromagnetic Ni0 clusters
are not active (Fig. 1).
Conclusions
For the first time, oligomerization of butenes (Raffinat III) has been
followed over supported Ni-containing catalysts by EPR under true
reaction conditions (20 bar, 80 °C). Initially formed ferromagnetic
Ni0 clusters revealed to be not active and are partially dissolved
during equilibration, most probably by reaction with H + to form
Ni + sites regarded as active ones. The experimental setup can be
eas-ily adapted to other reactions working with catalysts containing
paramagnetic transition metal species.
Literatur
[1] P. Albers, U. Bentrup, A. Brückner, F. Nierlich, H.-W. Zanthoff, D.
Maschmeyer, DGMK-Tagungsbericht 2004-3, ISBN 3-936418-23-3.
Operando-EPR of 6% Ni/
SiO -Al O under 20 bar
2 2 3
Raffinate III at 80 °C:
Ni0 cluster formation
completed within first
90 min (top) but activity
starts only after 300 min
(bottom)
→ Ni0 particles not active
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