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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
INTRODUCTION
EXPERIMENTAL PART
Selective oxidation of n-butane to maleic anhydride (MA) is the only commercially utilized chemical process for light Siliceous SBA-15 was synthesized via liquid crystal 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 60%VPO/SBA-15
the quantitative 1H MAS NMR studies further confirms the 20%VPO/SBA-15
Difference
Difference
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 n
=2.80 mmol g-1
60 wt.% VPO/SBA-15
n
=1.60 mmol g-1
Table 1. Physicochemical characterization of the SBA-15 and VPO loaded catalysts.
20%VPO/SBA-15
δ 51V /ppm
Figure 4. 51V MAS NMR spectra of the 20% and 60%VPO/SBA-15
n
=1.61 mmol g-1
catalysts. The spectra were recorded at 105.25 MHz using an excitation 60%VPO/SBA-15
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
Figure 1. 29Si and 1H MAS NMR spectra of SBA-15, 20%,
Table 2. 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 ca.1650
hemihydrate (VOHPO4 0.5 H2O) VPO precursor.
The activated catalysts show peaks characteristic of room temperature
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. 2600 ppm 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-butane δ31P/
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
δ13C/ppm
free induction decays were collected at the sample spinning of n-butane
rate of 10.0 kHz. Asterisks indicate spinning side bands. Figure 5. 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
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-butane 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 δ31 ppm
δ31 ppm
composed mainly of various orthophosphate phases. Initial Isovalent phase transformation from one orthophosphate phase Figure 3. 31P MAS NMR spectra of calcíned 60%VPO/SBA-
to another one is the reason for the change in the intensity n-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 n-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. Acknowledgements
Literature
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