Oxidative dehydrogenation of propane over rare-eath orthovanadates

Article abstract of DOI:10.1039/a607565g

High-purity rare-earth orthovanadates (R_EVO4), PrVO4, GdVO4, DyVO4, HoVO4, ErVO4, NdVO4, TbVO4 and LuVO4, have been prepared by the citrate method.XRD, FTIR, LRS, UV-VIS diffuse reflectance, TPR and EPR techniques have been employed to characterize them.The catalytic performances of PrVO4, ErVO4, GdVO4, DyVO4 and NdVO4 in the oxidative dehydrogenation of propane can compete with that of Mg3V2O8.The selectivity of propene over TbVO4, LuVO4 and HoVO4 was relatively low.TPR results showed that the more easily the catalyst is reduced, the higher the propene selectivity.EPR and in situ Raman experiments confirmed the presence of low-valence vanadates at the catalyst surface in the reaction process.We observed that 18O2-isotope exchange occurred over the catalysts via a double-step single exchange process and the activity of the catalysts increased with an increase of isotope exchange rate.

Full text of DOI:10.1039/a607565g

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Oxidative dehydrogenation of propane over rare-earth orthovanadates
Chak-Tong Aua* and Wei-De Zhanga,b
a Department of Chemistry, Hong Kong Baptist University, Kowloon T ong, Hong Kong
b Department of Chemistry, Xiamen University, Xiamen 361005, P. R. China
High-purity rare-earth orthovanadates (R VO ), PrVO , GdVO , DyVO , HoVO , ErVO , NdVO , TbVO and LuVO ,
E
4
4
4
4
4
4
4
4
4
have been prepared by the citrate method. XRD, FTIR, LRS, UVÈVIS di†use reÑectance, TPR and EPR techniques have been
employed to characterize them. The catalytic performances of PrVO , ErVO , GdVO , DyVO and NdVO in the oxidative
4
4
4
4
4
dehydrogenation of propane can compete with that of Mg V O . The selectivity of propene over TbVO , LuVO and HoVO
3
2 8
4
4
4
was relatively low. TPR results showed that the more easily the catalyst is reduced, the higher the propene selectivity. EPR and in
situ Raman experiments conÐrmed the presence of low-valence vanadates at the catalyst surface in the reaction process. We
observed that 18O -isotope exchange occurred over the catalysts via a double-step single exchange process and the activity of the
2
catalysts increased with an increase of isotope exchange rate.
The utilization of relatively abundant and cheap alkanes in
the chemical industry is always desirable. In the transform-
ation of alkanes into valuable chemicals, selective oxidation is
considered to be important.1 VMgO was Ðrst reported to be
active and selective in the oxidative dehydrogenation (OXD)
of ethylbenzene.2 Kung and co-workers discovered that the
VMgO catalyst was also e†ective in the OXD of propane,3h5
n-butane3,6,7 and cyclohexane8 and attributed the active
phase to magnesium orthovanadate (Mg V O ).9 Three
perature programmed reduction (TPR), in situ Raman,
NOÈTPD and 18O -isotope exchange methods were used to
2
probe the relationship between the catalytic activity and the
nature of the catalysts.
Experimental
Catalyst preparation
R VO (R \ Pr, Gd, Dy, Ho, Er, Nd, Tb, Lu) were prepared
E
4
E
3
2 8
by the citrate method.14 The starting materials were analytical
grade R (NO ) É 5H O (Aldrich) and NH VO (atomic
factors, viz. (i) the presence of tetrahedral VO , (ii) the absence
4
of VxO double bonds and (iii) the di†erence in metallic vana-
E
3 3
2
4
3
(atomic
ratio \ 1 : 1). R (NO ) É 5H O and NH VO
ratio \ 1 : 1) were Ðrst dissolved in deionized water and citric
acid was then added in such a manner that the molar number
of equivalent anions (three ions per molecule of citric acid)
dates, were pointed out to be responsible for the oxidative
dehydrogenation properties of VMgO catalysts.6,7,10 They
also conÐrmed that due to the di†erence in alkane molecular
structure and size, propane OXD to propene was active and
selective on Mg V O and a-Mg V O , whereas n-butane
E
3 3
2
4
3
equalled that of the cations (total amount of R3` and V5`).
E
3
2 8
2 2 7
The resulting solution was heated on a steam-bath to obtain
was only selective over Mg V O but not over a-Mg V O .9
3
2 8
2 2 7
the solid which was heated at 400 ¡C for 24 h (to decompose
the organic precursor) and then calcined at 550 ¡C for 6 h.
In contrast to the suggestion of Kung and co-workers, Volta
and co-workers and Guerrero-Ruiz et al. suggested that mag-
nesium pyrovanadate (a-Mg V O ) was the active and selec-
2
2 7
Catalyst characterization
tive phase, whereas Mg V O and MgV O were phases
3
2 8
2 6
responsible for total oxidation.11,12 They compared the cata-
The structures of the prepared catalysts were conÐrmed by
X-ray di†raction (XRD) using a Rigaku D/Max-RC instru-
ment equipped with Cu-Ka radiation. The speciÐc surface
area of the catalysts was measured by the BET method of
nitrogen adsorption at liquid-nitrogen temperature. FTIR
spectra were recorded between 400 and 1200 cm~1 with a
Nicolet Magna-IR 550 spectrometer. Laser Raman spectro-
scopic (LRS) studies were performed on a Jobin Yvon U-1000
Raman spectrometer. UVÈVIS di†use reÑectance spectra were
recorded between 380 and 780 nm with a Shimadzu UV-2100
UVÈVIS recording spectrophotometer. EPR was conducted
on a Bruker 200D-SRC meter.
lytic performances of pure a-Mg V O , Mg V O and
2
2 7
3 2 8
MgV O in propane OXD with the reducibility and surface
2
6
properties [characterized by XPS, EPR, electrical conductivity
and nitrogen oxide (NO)ÈTPD] of the catalysts, and proposed
that the presence of stable V4` ions and oxygen vacancies in
a-Mg V O are responsible for the high propene selectivity
2
2 7
over a-Mg V O . In their opinion, the corner-sharing tetra-
2 7
2
hedral VO structure in a-Mg V O is more favourable for
4
2 2 7
oxygen atom extraction than isolated tetrahedral VO or
4
octahedral VO structures existing respectively in Mg V O
and MgV O . Such a di†erence in structure has been corre-
lated to the catalytic performance of these three catalysts in
oxidative dehydrogenation reactions.
6
3 2 8
2
6
Temperature programmed reduction (TPR) was conducted
by using 7% H È93% N (v/v). The Ñow rate of the carrier gas
2
2
Considering the chemical properties of rare-earth and
alkaline-earth elements and with the aim to utilize rare-earth
elements in the Ðeld of catalysis, we studied previously the
pure phases of rare-earth orthovanadates R VO (R \ Y, Ce,
was 20 ml min~1 and a thermal conductivity detector was
used. 40 mg of sample was used and the heating rate was
10 ¡C min~1.
The NOÈTPD and oxygen isotope exchange experiments
were conducted in a Ñow system connected to a Hewlett
Packard G1800A GCD mass quadrupole spectrometer. For
the NOÈTPD study, 200 mg of each sample was pretreated in
a Ñow of pure He at 550 ¡C for 2 h and then cooled to 30 ¡C
E
4
E
La, Nd, Sm, Eu) for the OXD of propane.13 In this paper, we
report further the preparation and characterization of other
pure rare-earth orthovanadates R VO (R \ Pr, Gd, Dy, Ho,
E
4
E
Er, Nd, Tb, Lu) for the oxidative dehydrogenation of propane.
Techniques such as BET surface-area measurements, XRD,
IR, Raman, UVÈVIS di†use reÑectance spectroscopy and
EPR were employed to characterize the catalysts. Tem-
for NO adsorption. For the 18O -isotope exchange experi-
2
ments, the sample was cooled to 200 ¡C and 10 ll 18O (95È
2
98%) was pulsed into the reactor. The procedures were
J. Chem. Soc., Faraday T rans., 1997, 93(6), 1195È1204
1195

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repeated at 300, 400, 500 and 600 ¡C. 16O , 16O18O and 18O
2
2
were monitored at the exit of the reactor to detect the oxygen
exchange ability of each catalyst.
Catalyst performance
Catalytic testing was conducted in a Ñow system. The sample
(
0.050 g) with particle size 35È75 mesh was placed in a Ðxed-
bed (inner diameter 5 mm) quartz microreactor. Under stan-
dard conditions, the feed was composed of 10% O , 20%
2
C H and 70% He (v/v/v) and the total Ñow rate was 50 ml
3
8
min~1. An empty reactor showed no activity. All data were
collected after 4 h of reaction.
The reaction products were analysed by on-line gas chro-
matography (Shimazu GC-8A equipped with C-R6A data
processor) with helium as carrier gas. Two columns were used
in parallel. A Porapak Q (60È80 mesh, Aldrich) column was
used to separate the hydrocarbons and CO while molecular
2
sieves 5A were used to separate O and CO. The column tem-
2
perature was 100 ¡C. The conversion of propane was deÐned
as (mole of propane consumed/mole of propane in
feed) ] 100% and the selectivity of product A was deÐned as
(
mole of product A/mole of propane consumed) ] 100%/R
C
where R is the ratio of the number of carbon atoms in
propane to the number of carbon atoms in product A.
C
Results and Discussion
BET measurement
Fig. 1 XRD proÐles of R VO (from bottom to top: PrVO ,
The speciÐc surface areas of the prepared catalysts R VO are
E
4
4
E
4
GdVO , DyVO , HoVO , ErVO , NdVO , TbVO and LuVO )
listed in Table 1. The areas ranged from 12.2 to 44.8 m2 g~1.
The catalysts can be divided into two groups. The surface
areas of ErVO , GdVO and HoVO are higher ([32 m2
4
4
4
4
4
4
4
4
4
4
g~1) while that of the other Ðve are lower (\22 m2 g~1). The
di†erence in surface areas may a†ect the activity of the cata-
lysts.
X-Ray di†raction
Fig. 1 shows the XRD proÐles of the R VO catalysts. The
E
4
results showed that high-purity tetragonal (zircon structure)
PrVO , GdVO , DyVO , HoVO , ErVO , NdVO , TbVO
4
4
4
4
4
4
4
and LuVO were obtained and no other phases were detected.
4
The XRD spectra are in good agreement with the standard
spectra of PrVO ,15 GdVO ,16 DyVO ,17 HoVO ,18
4
4
4
4
ErVO ,19 NdVO ,20 TbVO 21 and LuVO .22 The d data
4
4
4
4
hkl
of the prepared R VO compared well with those of standards
E
4
listed in the ASTM Ðles (Table 2). The XRD line intensity of
TbVO4 is lower than those of the other catalysts, implying
that crystallization of the compound was poor during prep-
aration.
FTIR study
Fig. 2 shows the FTIR spectra of the prepared R VO
samples. For all the R VO studied, the strongest IR absorb-
E
4
E
4
ance peak appears around 790È830 cm~1. The strongest IR
peaks for GdVO , DyVO and NdVO are close to each
4
4
4
other (ca. 800 cm~1). The most intense IR peak of PrVO is at
4
7
97 cm~1 while that of LuVO is at 823 cm~1. All the cata-
4
lysts show two very weak peaks at 419 and ca. 450 cm~1. The
IR peaks and their assignments are given in Table 3. We
noticed that there is a very weak peak at about 1020 cm~1 in
the IR spectra of PrVO , DyVO and NdVO , which might
Fig. 2 FTIR spectra of R VO (from bottom to top: PrVO ,
4
4
4
E
4
4
be due to the presence of V O .23
GdVO , DyVO , HoVO , ErVO , NdVO , TbVO and LuVO )
2
5
4
4
4
4
4
4
4
Table 1 SpeciÐc surface areas of R VO
E
4
catalyst
PrVO4
13.8
GdVO4
43.1
DyVO4
14.6
HoVO4
32.8
ErVO4
44.8
NdVO4
12.2
TbVO4
18.7
LuVO4
21.8
surface area/m2 g~1
1196
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J. Chem. Soc., Faraday T rans., 1997, V ol. 93
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Table 3 Rare-earth orthovanadate FTIR peaks and their assigments24
PrVO4
GdVO4
DyVO4
HoVO4
ErVO4
NdVO4
TbVO4
LuVO4
assignment
4
4
7
19 vw
46 w
97 vs
419 vw
452 w
804 vs
419 vw
447 w
801 vs
419 w
454 w
812 vs
897 (sh)
419 vw
447 w
800 vs
419 vw
453.5 w
810 vs
904 (sh)
455 w
823 vs
895 (sh)
l (VO )
S
4
(VO )
4
808 vs
l
AS
9
01 (sh)
d (VO )
S
4
1
022 w
1018 vw
1025 w
d(VxO)
Raman study
vanadates24,25 and no V O was detected in the XRD spectra,
2
5
we believe that the amount of V O impurity in the prepared
The Raman spectra of the eight rare-earth orthovanadates are
presented in Fig. 3 and the peaks are tabulated in Table 4.
Compared with the other samples, the Raman peak of TbVO
2 5
PrVO , DyVO , HoVO and NdVO catalysts is extremely
4
4
4
4
small.
UV–VIS di†use reÑectance spectra
UVÈVIS spectra of the R VO samples have been recorded
4
is very weak. Similar to the Raman spectrum of Mg V O ,24
3
2 8
which has a very strong peak at 861 cm~1, the spectra of
PrVO , GdVO , DyVO , HoVO , ErVO , NdVO and
E
4
4
4
4
4
4
4
4
(
Fig. 4). There are intense bands within the 550È800 nm range
LuVO also show a strong peak within the range 870È910
in the spectra of all the samples. Except for GdVO , TbVO
cm~1. The spectrum of TbVO only shows a weak peak at
4
4
4
and LuVO , the spectra are split into two or three signals.
8
88 cm~1. PrVO has another peak at 1000 cm~1, while
4
4
PrVO and DyVO show a strong peak at 584 nm and a
HoVO and NdVO have two peaks at around 706 and 1000
cm~1, indicating that V O was present as impurity.23 Since
the Raman cross-section of V O is much larger than those of
4
4
4
4
weak peak at 450 nm. HoVO and ErVO present a band at
ca. 640 nm and three peaks at 513, 485 and 400 nm. NdVO
4
4
2
5
4
2
5
has a band at 731 nm and two peaks at 627 and 568 nm. The
charge-transfer bands of vanadium (both V5` and V4`) are
reported to be in the range 300È500 nm,26,27 while a typical
dÈd transition of V4` is in the range 700È800 nm. In our
spectra, the bands within the 300È500 nm range are weak,
while the bands within the 700È800 nm range are very strong.
They may be attributed to V4` and rare-earth ions.
Catalyst performance
The catalytic performances of the eight rare-earth vanadates
in the oxidative dehydrogenation of propane were monitored.
The major products were propene, CO and CO ; no oxygen-
2
ated product was observed. The carbon balance was found to
be 97.5È101%. Table 5 shows the results of the catalytic OXD
of propane at 500 ¡C. The results indicated that, except for
TbVO and ErVO , the R VO catalysts were quite selective
4
4
E
4
towards propene. The conversion of propane varied between
1
7
.85 and 35.9%. The selectivity of propene ranged from 8.69 to
8.3%. ErVO showed the highest propane conversion
4
(35.9%), while NdVO the lowest (1.85%). The GdVO cata-
4
4
lyst was both active and selective and the propene yield was
high under the adopted testing condition. The TbVO catalyst
4
was the least selective of the eight catalysts. Since the conver-
sion of propane over TbVO was too low to be tested at a
4
space velocity of 60 000 ml h~1, a space velocity of 30 000 ml
h~1 was adopted. The propane conversion over GdVO ,
4
HoVO and ErVO was relatively higher than that over the
4
4
Fig. 3 Raman spectra of R VO (from bottom to top: PrVO ,
other catalysts. This could be attributed to their high speciÐc
surface areas (Table 1). In order to present the catalytic behav-
E
4
4
GdVO , DyVO , HoVO , ErVO , NdVO , TbVO and LuVO )
4
4
4
4
4
4
4
Table 4 Raman peaks of rare-earth orthovanadates
PrVO4
GdVO4
DyVO4
HoVO4
ErVO4
NdVO4
TbVO4
LuVO4
1
48 s
2
2
3
4
4
5
64 w
264 m
392 m
484 w
284 vw
318 vw
392 w
434 w
504 vw
640 vw
264 m
380 w
88 vw
78 vw
12 vw
76 vw
28 vw
386 w
408 vw
476 vw
528 vw
6
7
64 vw
08 vw
706 w
798 m
812 m
874 s
7
8
8
94 m
08 m
70 m
812 m
830 m
888 s
802 m
818 m
882 s
822 w
842 m
898 s
822 w
838 m
896 s
832 w
850 m
904 s
888 w
1
000 w
1000 w
1000 m
1096 vs
1198
J. Chem. Soc., Faraday T rans., 1997, V ol. 93

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Fig. 5 Relation of propene selectivity and the conversion of propane.
Conditions: catalyst weight \ 0.05 g, T \ 500 ¡C, C H : O :
3
8
2
He \ 20 : 10 : 70, total Ñow rate varied from 20 to 130 ml min~1. The
activity and selectivity of TbVO were too low to be shown.
4
another small peak below 600 ¡C. We consider the peaks
between 680 and 760 ¡C to be due to the reduction of V5` to
V3`. The peaks of lower intensity below 600 ¡C could be due
to the reduction of surface species as XRD studies of the
samples reduced up to 600 ¡C revealed no change in crystal
structure. As conÐrmed by XRD, R VO was reduced to
E
4
R VO after TPR treatment (Fig. 7 and Table 6). The XRD
E
3
results also revealed that no apparent reduction of R
E ions
had occurred in the TPR process.
The di†erent catalytic actions of the orthovanadates can be
related to their redox properties. It has long been postulated
that the catalytic activity and selectivity of an oxide in selec-
tive oxidation can be related to the reduction rate of the
oxide. Over an oxide which is very difficult to reduce, the
activity will be low; conversely for an oxide which is very easy
to reduce, the activity is high but the selectivity low. An active
and selective catalyst should have an intermediate ease of
reduction.28 Comparison of the propene selectivity over the
catalysts (Fig. 5) with the temperature of the main TPR peaks
Fig. 4 UVÈVIS spectra of R VO (from bottom to top: PrVO ,
E
4
4
GdVO , DyVO , HoVO , ErVO , NdVO , TbVO and LuVO )
4
4
4
4
4
4
4
iour of the catalysts clearly, the selectivity as a function of
conversion is plotted in Fig. 5. This shows that at similar
propane conversion, the selectivities over the PrVO , ErVO ,
(Fig. 6), leads to the observation of a correlation. The
4
4
GdVO , DyVO and NdVO catalysts are similar, and are
4
4
4
higher than that of TbVO , LuVO and HoVO . The cata-
lytic behaviour of the more active orthovanadates is compara-
4
4
4
ble with that of Mg V O .25
3
2 8
Temperature programmed reduction (TPR)
In order to examine the reducibility of the rare-earth ortho-
vanadates, TPR experiments were conducted and the results
are shown in Fig. 6. One can see that all the TPR proÐles have
a signiÐcant peak positioned within the range 680È760 ¡C and
Table 5 Performance of rare-earth orthovanadates in propane OXD
at 500 ¡C
selectivity (%)
catalyst C H
8 conv. (%)
C H
CO2
CO C H
6 yield (%)
3
3
6
3
PrVO4
2.17
7.28
4.35
12.4
35.9
1.85
9.31
6.62
78.3
61.5
67.5
45.0
22.4
76.9
7.70 14.0
1.70
4.48
2.94
5.58
8.04
1.42
0.08
3.54
GdVO
4
16.0
22.5
16.2
35.5
55.0
DyVO4
16.3
19.5
22.6
HoVO
4
ErVO4
NdVO
9.79 13.3
4
TbVO a
8.69 60.0
53.5 17.6
31.3
28.8
4
LuVO b
4
Space velocity \ 60 000 ml g~1, except for a 12 000 ml g~1 and
b 30 000 ml g~1.
Fig. 6 TPR proÐles of R VO (from bottom to top: PrVO ,
E
4
4
GdVO , DyVO , HoVO , ErVO , NdVO , TbVO and LuVO )
4
4
4
4
4
4
4
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
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Fig. 8 In situ Raman spectra of DyVO treated at (a) 500 ¡C, N , (b)
4
2
room temperature, O , (c) 200 ¡C, 10O È20C H È70N , (d) 500 ¡C,
2 2 3 8 2
1
0O È20C H È70N , 15 min, (e) 500 ¡C, 10O È20C H È70N , 1 h
2
3
8
2
2
3
8
2
and (f) 500 ¡C, 10O È70N , 15 min
2
2
Fig. 7 XRD proÐles of R VO after TPR (from bottom to top:
oxidation reaction, vanadium-based catalysts show V5`ÈV4`
and V4`ÈV3` redox couples.32 In the reaction of propane
oxidative dehydrogenation over orthovanadates, we cannot
conÐrm conclusively which couple is responsible for the
dehydrogenation, but the low-valence V present at the surface
of the catalysts seems to play an important role in the
oxidative dehydrogenation process.
E
4
PrVO , GdVO , DyVO , HoVO , ErVO , NdVO , TbVO
4 and
4
4
4
4
4
4
LuVO ), indicating R VO formation
4
E
3
reduction peaks of GdVO , ErVO , DyVO , PrVO and
4
4
4
4
NdVO occurred at lower temperature than those of TbVO ,
4
4
LuVO4
and HoVO . The selectivity of propene over the
The Raman signal of O 2~ species is suggested to be within
4
2
former Ðve catalysts was higher than that over the latter three,
indicating that, in the present cases, the easier the reducibility
of the orthovanadates, the higher the selectivity.
the range 730È950 cm~1 while that of O ~ is at ca. 1164
2
cm~1.33,34 We could not identify signals due to O 2~ as there
2
are vanadate peaks present within the 730È950 cm~1 region.
We did not observe any peak due to O ~ during the reaction.
2
In situ Raman study
EPR and NO–TPD studies
In situ Raman experiments were performed to investigate
changes in the catalysts after the oxidation reaction. Fig. 8
Fig. 9 shows the EPR spectra of the R VO catalysts and
clearly reveals that the V4` ions were present in the catalysts.
The NdVO , HoVO and TbVO samples show an intense
E
4
shows the Raman spectra of the DyVO catalyst under
4
di†erent reaction conditions. The peak at about 875 cm~1 is
4
4
4
attributed to VO , while the peaks at 798 and 814 cm~1
signal with a g value of about 1.98 which was assigned to V4`
4
correspond to MÈOÈV.24,29h31 The spectra do not noticeably
ions.27 The PrVO , DyVO and ErVO samples show hyper-
4
4
4
change when the catalyst was treated up to 500 ¡C under an
Ðne structure due to V4` (S \ 1/2; I \ 7/2) in a distorted
tetrahedral structure. The hyperÐne structure is not particu-
larly well resolved indicating that the V4` ions in the catalysts
are not very far apart from each other. In other words, the
concentration of V4` ions in the catalysts is not low. The V4`
atmosphere of N or O (the peak at ca. 1000 cm~1 is due to
2
2
Under
V O
impurity).
the
reactant
mixture
2
5
1
0O È20C H È70N at 200 ¡C [Fig. 8(c)], the spectra did not
2
3 8
2
change; however, after reaction at 500 ¡C for 15 min, the
peaks at 798, 814 and 875 cm~1 decreased in intensity. They
were weakened further after 1 h reaction, indicating that the
surface species had changed. The original spectrum could be
restored by stopping the propane in the feed. The results
clearly show that the vanadyl species on the catalyst surface
had been reduced in the reaction process. We noticed that at
the end of the reaction, the original colour of the catalysts
ions in the catalysts can be easily oxidized by O . Fig. 9(g)
shows the EPR spectrum of ErVO treated by O at 550 ¡C
2
2
4
for 10 min and then sealed in a glass tube. The spectrum
shows that the EPR signal at g \ 1.98 was greatly reduced.
The NOÈTPD method can also be used to reveal the pres-
ence of V4` ions.12 If there are V4` ions in the catalysts,
chemically adsorbed NO can react with them at elevated tem-
peratures. Two NO molecules react with a V4` ion to
produce one molecule of N O. Since the desorption of N O
(m/z \ 44) can be masked by CO , we Ðrst characterized the
CO desorption proÐle of the catalyst. Fig. 10(a) shows the
CO spectrum of the fresh ErVO sample. There is a small
shoulder at ca. 390 ¡C and the threshold for major CO
changed from white or yellowish white (except for NdVO
which was greenish grey originally) to grey. This implies that
4
2
2
the valence of V at the surface of the catalysts has changed
from ]5 to lower valence(s) (]4 or even ]3). The XRD
proÐles of the catalysts after reaction nevertheless indicate
that the rare-earth orthovanadates preserved their original
structures. In other words, the bulk of the catalyst was stable
during the reaction. The amount of low oxidation state
vanadium was very small (not detectable by XRD). In the
2
2
2
4
2
desorption is at 520 ¡C. Fig. 10(b) shows the NOÈTPD of
ErVO pretreated by He at 550 ¡C for 2 h. The peak at ca.
4
440 ¡C corresponded to the release of N O when NO reacts
2
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J. Chem. Soc., Faraday T rans., 1997, V ol. 93
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admission of 18O . The observed isotope exchange must have
2
occurred either via consecutive exchanges of gas-phase oxygen
(
18O ) with lattice oxygen (16O) or through a single-step
2
multiple exchange procedure.
Fig. 11 shows the result of 18O -isotope exchange over
2
GdVO and ErVO . It shows that the threshold temperature
4
4
for the exchange of gaseous with lattice oxygen was about
4
20 ¡C. At any temperature above 420 ¡C, the amount of 16O
2
was much higher than that of 16O18O, signifying that double
exchange was dominant. The amount of 16O increased lin-
2
early with increasing temperature while that of 16O18O was at
a maximum at 500 ¡C and then decreased.
The double exchange of oxygen may be achieved by means
of double-step single exchange:
1
8O18O(g) ] o 16O o \ 18O16O ] o 18O o
8O16O(g) ] o 16O o \ 16O16O ] o 18O o
1
or single-step double exchange:
18O18O(g) ] 2 o 16O o \ 16O16O ] 2 o 18O o
Here o 16O o or o 18O o represents lattice oxygen. It is considered
that the 18O -isotope exchange is a result of thermo-
2
equilibrium. We notice that the extent of double exchange is
much larger than that of single exchange and the curve of
single exchange exhibits a maximum at ca. 500 ¡C. The extent
of double exchange reached approximately 100% at 600 ¡C.
From these results, we deduce that double-step single
Fig. 9 EPR spectra of (a) TbVO , (b) NdVO , (c) HoVO , (d)
4
4
4
PrVO , (e) DyVO , (f) ErVO , and (g) ErVO pretreated with O
2 at
4
50 ¡C
4
4
4
5
exchange is the main process for 18O -isotope exchange over
2
the catalysts.
Based on the EPR, NOÈTPD, TPR, in situ Raman and
with V4`. On the other hand, the NOÈTPD of ErVO
4
pretreated by O at 550 ¡C for 1 h has no peak in this range,
2
indicating that the V4` ions have been oxidized by O in the
2
pretreatment process. This result is in good agreement with
the EPR results.
1
8O -isotope exchange
2
1
8O has been employed to determine the degree of isotope
2
exchange between gas-phase and lattice oxygen of the cata-
lysts.35 In our present study, the results obtained can be
related solely to the exchange between gas-phase and lattice
oxygen (so-called heterophase exchange), since blank experi-
ments (without catalyst in the reactor) showed no occurrence
of isotope exchange in the temperature range (200È600 ¡C)
adopted in our investigation. In the experiments, catalysts
were Ðrst pre-treated with pure He at 600 ¡C for 2 h before the
Fig. 10 NOÈTPD of ErVO : (a) CO spectrum of a fresh sample,
4
2
(
b) N O spectrum, sample pretreated with He at 550 ¡C for 2 h and (c)
Fig. 11 Result of 18O -isotope exchange over (a) GdVO
4 and (b)
2
2
N O spectrum, sample pretreated with O at 550 ¡C for 1 h
ErVO
2
2
4
1202
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veniently by the citrate method. No impurity was detected by
XRD in all of the eight prepared catalysts; the more sensitive
IR and Raman techniques, however, showed that there was a
small amount of V O impurity in the PrVO , DyVO ,
2
5
4
4
HoVO and NdVO catalysts. The prepared R VO catalysts
4
4
E
4
were active and selective in the oxidative dehydrogenation of
propane. The catalytic performances of PrVO , ErVO ,
GdVO , DyVO and NdVO in the oxidative dehydroge-
4
4
4
4
4
nation of propane are similar to that of Mg V O . The cata-
3
2 8
lytic performance of the rare-earth orthovanadates can be
related to the redox properties as well as to the surface
properties of the catalysts. EPR, NOÈTPD, in situ Raman and
Scheme 1
catalytic performance results, we propose that there are
1
8O -isotope exchange results strongly indicate that the low-
2
oxygen vacancies and a very small amount of V4` in the cata-
valence vanadates present were playing an important role in
the dehydrogenation process.
lysts. The 18O molecule may react with the catalysts as
2
shown in Scheme 1, where [e] represents an oxygen vacancy
in the lattice of the catalysts. Since the amount of o 16O o was
much larger than that of o 18O o , the amount of single-
exchange product 16O18O remained lower than that of double
exchange. The single-exchange product increased initially due
to the accumulation of exchanged 18O in the lattice. However,
upon an increase in the exchange rate with increasing tem-
perature, there was a decline in single-exchange product above
The authors gratefully acknowledge the support of this work
by the Hong Kong Research Grants Council, UGC (HKBC
102/93E) and the State Key Laboratory for Physical Chem-
istry of Solid Surface in Xiamen University, P. R. China, for
the facilitation of the LRS, UVÈVIS spectra and EPR
measurements.
520 ¡C.
From the analyses of the isotope exchange kinetics, one can
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1203

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Paper 6/07565G; Received 6th November, 1996
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J. Chem. Soc., Faraday T rans., 1997, V ol. 93