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Vanadium phosphates on mesoporous supports:
Model catalysts for solid-state NMR studies of the
selective oxidation of alkanes
J. Frey, Y. S. Ooi, B. Thomas, R.V. Reddy Marthala, A. Bressel, M. Hunger
Institute of Chemical Technology, University of Stuttgart, 70550 Stuttgart, Germany
Selective oxidation of n
-butane to maleic anhydride (MA) is the only commercially utilized chemical process for light
Siliceous SBA-15 was synthesized via
alkanes. Bulk vanadium phosphorous oxide (VPO) is the catalyst commonly used for this reaction.1 VPO catalysts
template (LCT) method. Mixed (iso-butyl/benzyl) alcohol
are composed of a large number of phases, such as αI-, αII-, β-, γ-, and δ-VOPO4 (vanadyl orthophosphate, V5+),
route was adopted as the preparation media for the
vanadyl pyrophosphate (VPP, V4+), VPO4 (V3+) etc. For a long time, is was accepted that (VO)2P2O7 is the main effective dispersion of VPO on to SBA-15.
component of the active catalyst. Till date, however, little is known about the exact nature of the active sites.1-5
The hemihydrate VPO precursor (VOHPO4 0.5 H2O) was
Supported catalysts offer several advantages, such as (1) higher surface area to volume ratio of the active phase,
activated in air/butane/nitrogen mixture at 673 K for
(2) high mechanical strength, (3) improved heat transfer characteristics, and (4) controllable catalyst texture.2,3,5
16 h. Activated catalysts were characterized by chemical
In the case of VPO catalysts, strong support-oxide interactions can hinder the formation of the pyrophosphate
analysis (ICP-OES), X-ray diffraction (D8 Advance),
phase or cause changes in the phase composition affecting the n
-butane conversion and/or MA selectivity.
nitrogen adsorption (ASAP2000), and solid-state NMR.
The resulting material generally consists of phases that resemble, e.g. α-VOPO4 or γ-VOPO4.2,4
All NMR studies were performed on a Bruker MSL-400
In the present study, siliceous SBA-15 was used as support for VPO compounds to take advantage of the efficient
spectrometer using 7 mm (29Si) and 4 mm MAS probes (1H,
dispersion of catalytically active components.
RESULTS AND DISCUSSION
Analysis of the side band intensities of 51V MAS NMR
Si MAS NMR
spectra (Fig. 4) gives chemical shift anisotropies
The 29Si MAS NMR spectra (Fig. 1) reveal characteristic signals
corresponding to vanadium in distorted octahedral
of Q2, Q3, and Q4 silicon species. A clear dependence of the
environments.6-7 Large values of CSA indicates a very short
number of SiOH groups on the VPO loading as determined by
the quantitative 1H MAS NMR studies further confirms the
Decrease in surface area upon VPO loading on SBA-15.
The small angle XRD patterns show characteristic (100), (110)
20 wt.% VPO/SBA-15
δ 29Si /ppm
δ 29Si /ppm
and (200) reflections, but with an attenuation in intensity with
1H MAS NMR
increasing VPO loading. The wide-angle patterns exhibit lines
=2.80 mmol g-1
60 wt.% VPO/SBA-15
=1.60 mmol g-1
Physicochemical characterization of the SBA-15 and VPO loaded catalysts.
δ 51V /ppm
51V MAS NMR spectra of the 20% and 60%VPO/SBA-15
=1.61 mmol g-1
catalysts. The spectra were recorded at 105.25 MHz using an excitation
pulse of 0.61 μs with 500 ms repetition time. Approximately 20,000 free
induction decays were collected with the sample spinning rate of 10.0 kHz.
δ 1H /ppm
29Si and 1H MAS NMR spectra of SBA-15, 20%,
Parameters obtained by simulation of the 51V MAS NMR spectra.
60%VPO/SBA-15 catalysts recorded at room temperature.
The spectra were recorded at 79.49 and 400.13 MHz using
excitation pulses of 5.0 and 4.0 μs, respectively. 180 and 64
free induction decays were collected for each spectrum with
repetition times of 30 and 10 s, respectively.
non-activated precursor (VOHPO 0.5 H O/SBA-15)
P MAS NMR spectrum (Fig. 2) of non-activated VPO/SBA-15
exhibits a broad peak at ca.
1650 ppm typical of the
hemihydrate (VOHPO4 0.5 H2O) VPO precursor.
The activated catalysts show peaks characteristic of
activated/non-rehydrated activated/rehydrated/dehydrated at 723 K
phosphorus in the vicinity of V5+ (orthophosphate phases)
centers. The absence of broad signals at ca.
indicates that no vanadyl pyrophosphate phases were formed.
It is well-known that VPO compounds dispersed on silica
2.8 to -22.3
heated at 523 for 20 minutes
Maleic anhydride (2,5-furandione) is produced from n
heated at 573 for 20 minutes
in a 14-electron oxidation involving the abstraction of 8 Figure 2.
31P MAS NMR spectra of the non-activated
hydrogen atoms and insertion of 3 oxygen atoms. It is the most precursor, activated/non-rehydrated, and activated/
rehydrated/dehydrated 60%VPO/SBA-15 catalyst. The
complex selective oxidation reaction industrially practiced.
spectra were recorded at 161.98 MHz using an excitation
Generally, bulk VPO material is the catalyst used for this pulse of 0.61 μs with 30 s repetition time. Approximately 320
CO MA MA CO olefinic
free induction decays were collected at the sample spinning
rate of 10.0 kHz. Asterisks indicate spinning side bands.
13C MAS NMR spectra of calcined 60%VPO/SBA-15 catalyst
loaded with ca.
120 mbar n
-butane-13C at room temperature and heated at
523 K and 573 K for 20 minutes. The spectra were recorded at 100.61 MHz
non-rehydrated:120 mBar n-butane; RT
4H12 + 3.5 O2
using an excitation pulse of 4.5 μs with 10 s repetition time. Approximately
8,000 free induction decays were collected at a spinning rate of 8.0 kHz.
31P MAS NMR signals (Fig. 3) characteristic of phosphorus
In this study, VPO catalysts supported on siliceous SBA-15
before and after catalytic oxidation of n
-butane at 573 K.
were prepared and ex situ
MAS NMR studies of n
Changes in the signal intensities occurred upon performing the
oxidation to maleic anhydride performed. The
20%VPO/SBA-15 and 60%VPO/SBA-15 catalysts were
composed mainly of various orthophosphate phases. Initial
Isovalent phase transformation from one orthophosphate phase
31P MAS NMR spectra of calcíned 60%VPO/SBA-
to another one is the reason for the change in the intensity
-butane-loaded samples at 523 and
15 catalyst recorded upon loading with 120 mbar 573 K by 13C MAS NMR spectroscopy confirmed the
distribution of the 31P MAS NMR signals.5
-butane and heating at 523 and 573 K. The spectra were
formation of MA. 31P MAS NMR studies of the spent
recorded at 161.98 MHz using an excitation pulse of 0.61 μs
During the oxidation reaction, a phase transformation from δ-
with 30 s repetition time. Approximately 320 free induction
catalysts show changes in the phase composition upon the
decays were collected at a spinning rate of 10.0 kHz.
Financial support by Deutsche Forschungs-
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