Calcination of VPO precursor
Kamilla M. S. Hansen
Abstract
The precursor for vanadium phosphate (VPO) catalyst was calcined at different temperatures and time
in a thermogravimetric analyzer (TGA). This was done to study the weight change during the
calcination process. The off gas from the TGA was led to a mass spectrometer (MS) to be analysed.
From the scientific and patent literature it was found that a large range of temperature (between 240
and 880ºC) and time (between 0.5 and 168h) were used in the calcination process. The temperatures
and times used for the experiment were decided on the basis of the literature study.
The results shows that absorbed water was desorbed between 80-100°C during the heating and
crystalline water exited the catalyst from about 260°C with a maximum weight loss at about 375°C.
The organic compounds in the precursor burned because of the presence of oxygen in the TGA and the
MS detected an increase in CO2 at a temperature about 260ºC with a maximum at about 440ºC. When
the precursor was heated to 480ºC and kept there for 10h, the calcined catalyst had change colour when
compared to the catalyst treated at 360ºC for 10h. The explanation to the colour change is properly that
vanadium change oxidation number from 4+ to 5+.
Introduction
Vanadium phosphorous oxide (VPO) is used to catalyze the partial oxidation of n-butane to maleic
anhydride. Maleic anhydride is the starting material for some commercially important chemicals such
as tetrahydrofuran, fumaric acid and malic acid. In the process of developing a catalyst that is highly
selective and active, the calcination/activation is an important step. The scientific and patent literature
has described dozens of procedures but a detailed analysis of the different methods and their effect on
the VPO catalyst has not been carried out. The starting material is a precursor with the chemical
formula: VOHPO4·½H2O. In the work the literature is reviewed to assess the different methods that are
used to calcine and activate VPO catalyst. On the basis of the literature review a set of experiments was
proposed to compare the used temperature and atmosphere. The experimental work was carried out on
a TGA and analysis of the off gas was done by MS. There is no previous example of using a TGA to
study the calcination process. The most used reactor type is a fix bed reactor, but Wang et al. [1-3],
Page 1 of 16
Richter et al. [4] and Sakakini et al. [5] used a microbalance reactor or a quadrupole mass spectrometer
to study the activation process or to kinetics study.
Experimental
The Calcination Procedure
The precursor (20 mg) was calcined in a TGA (Mettler Toledo, TGA/SDTA851e), which measured the
weight every 1 - 4 seconds for the experiments depending on the length of the experiment. The
atmosphere in the furnace was controlled as well as flow of gas through it (50 ml/min). The gas outlet
was connected to a MS (Pfeiffer Vacuum, ThermoStar GSD 300 T) and the frequency of measurement
was 24 seconds. After the sample was put into the TGA, argon was supplied to flush the air out of the
furnace for 15 min before the experiment was started. The precursor was heated with the rate of
4°C/min from 25ºC to the wanted temperature (260, 390 or 480°C) and hold there for 1, 4 or 10 hours.
During the heating an argon atmosphere was used which was changed to air in the static period. In one
experiment argon was used in both periods. All the data from the TGA and the MS were collected by a
computer.
The precursor is delivered by DuPont. The argon gas used was Argon HP+ and the air was compressed
air, both from Air Liquid.
The experimental parameters are shown in Table 1.
Table 1: The experimental parameters.
Heating
Exp. no.
1
2
3
4
5
6
7
8
9
10
Flow
ml/min
50
50
50
50
50
50
50
50
50
50
Temperature
ramp, ºC/min
4
4
4
4
4
4
4
4
4
4
Gas
Time
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
59min
1h32
1h54
59min
1h32
1h54
59min
1h32
1h54
1h54
Temperature
ºC
260
390
480
260
390
480
260
390
480
480
Static
Time
h
1
1
1
4
4
4
10
10
10
10
Gas
Air
Air
Air
Air
Air
Air
Air
Air
Air
Ar
Page 2 of 16
Results
Literature study
In the study of the scientific literature, it is found that many different temperatures and times have been
used to calcine and/or activate the VPO catalyst. In the patent literature the used temperature and time
interval is smaller. The values found are shown in the following figures (Figure 1 a. – 1 d.)
a.
b.
c.
d.
Figure 1: a and b shows the frequency of temperature and time during calcination, respectively, and c and d shows
the frequency of temperature and time during activation respectively [1-67].
The frequency of time is depicted in Figure 1 a. and 1 c. It is seen that there are a large spreading in the
time used in the scientific literature. When long time is used it is often because a specific phase of
(VO)2P2O7 is desired to be formed. In the patent literature, the time used is more generally about 1, 3,
and 6 hours for calcination. The frequency of used temperature is depicted in Figure 1 b. and 1 d. Here
the spreading is large in both the scientific and the patent literature. It is seen that there is a larger
Page 3 of 16
temperature range for the calcination than activation. Herrmann et al. [40] studied the effect of the
temperature during calcination and used the following temperature 450, 500, 600, 750 and 880ºC. The
three highest temperature is not used industrial and the aim was to produce single phase (VO)2P2O7.
The XRD showed a change in the reflection that most likely was a result of an increasing structural
disorder when the temperature decreases from 880 to 450°C.
The result of the study of which atmosphere was used during calcination is depicted in Figure 2. There
is a good agreement by using either air or nitrogen. It is only in the scientific literature that 100%
oxygen is used during the calcination. The aim of the oxygen treatment is to form - and -VOPO4
phases, which one that is formed depends on the temperature and time. Some authors have investigated
the effect of adding steam to the air or nitrogen. Richter et al. [4] report that a water vapour treatment
caused an increase in the P/V ratio on the surface, but also that the P/V ratio decreased again when the
VPO catalyst was exposed for a stream of dry nitrogen. Furthermore it was found that the enrichment
was influenced by the temperature. The P/V ratio decrease by increasing temperature.
Calcination
Literature
30
Patent
7
Frquency
20
4
20
10
15
3
4
0
Oxygen
Air
3
4
Air + Water
1-10% Oxygen
in Nitrogen
Nitrogen
Figure 2: The frequency of gas used during calcination fund in the
scientific and patent literature.
Both in the scientific and the patent literature it is normal to use butane in air or in a mixture of oxygen
and an inert gas such as nitrogen and helium during the activation. The concentration of butane varies
between 1 to 50% in the patent literature and between 0.75 to 2.8%, often approximately 1.5% in the
Page 4 of 16
scientific literature. When working with butane in air it should be notice that the explosion limits is
between 1.8 - 8.4%.
Initial tests
Initial tests were preformed to establish the amount of precursor, the flow and the heat ramp. The
experimental parameters are shown in table 2.
Table 2: The parameters for the test
Test no.
1
2
3
4
Flow
ml/min
45
45
6
6
Temperature
ºC
480
480
480
480
Gas
Ar
Ar
Ar
Ar
Heat ramp
ºC/min
20
20
20
4
Time
H
3
3
3
1
Peak 1
ºC
105.9
102.8
112.7
103.2
Peak 2
ºC
421.8
406.1
424.9
395.0
Figure 3 shows the differentiated weight loss during the heating. As seen there are two peaks. The first
peak (about 100°C) is due to the loss of absorbed water and the second to loss of crystal water when the
precursor is transformed to catalyst or from burning of organic compounds in the precursor.
Temperature
Test 1
Test 2
Test 3
600
0.03
0.025
0.02
400
0.015
300
0.01
0.005
200
d(%wt)/dt
Temperature (°C)
500
0
100
-0.005
0
-0.01
0
5
10
15
20
25
Tim e (m in)
Figure 3: The results from the tests with 20ºC/min as heat ramp.
It is furthermore seen that the maximum of the peaks are not at the same temperature. The temperature
of each maximum is shown in Table 2. In test 1-3 it seems like the heat ramp is too high, particular
when 50mg precursor is used. At the lower heat ramp particular the second peak has its maximum at a
lower temperature indicating that when using 20ºC/min the heating was too fast.
Page 5 of 16
During the tests with low flow the MS detected nitrogen. The explanation is that the low flow could not
maintain a pressure above the atmosphere in the TGA resulting in intake of air in the furnace.
On the basis of the four tests experiments it has been chosen to only use 20 mg precursor, a flow at 50
ml/min and a heat ramp at 4ºC/min.
Experimental results
It was found that flushing for 15 min with argon was not enough time to remove the oxygen which
leads to oxygen contamination during the heating. Because of the oxygen the organic compound in the
sample were burned and CO2 and H2O were formed. CO were probably also formed, but it was not
possible to measure it with the TGA because nitrogen was presence and it has the same molecular
weight as CO.
Exp 3
Exp 6
Exp 9
Exp 10
MS Intensity
1.00E-08
1.00E-09
1.00E-10
0
50
100
150
200
Tim e (m in)
Figure 4: The amount of CO2 in the off gas measured with a MS.
The temperature where the MS starts detecting an increase in the amount of CO2 is about 260ºC. In the
experiment where only argon is used, the peak is seen as a smooth and broad peak (See Exp 10 in
Figure 4). In the other experiments the peaks are more or less in two parts. This is caused by the change
in gas from argon to air which increase the amount of O2 in the furnace. For experiment 10 the
maximum CO2 peak is at 439ºC and for experiment 6 and 9 the maximum of the second peak is about
10 min after shifting to air.
Page 6 of 16
Exp 7
Exp 8
Exp 9
Exp 10
MS Intensity
1.00E-08
1.00E-09
0
50
100
150
200
Tim e (m in)
Figure 5: The amount of water in the off gas measured with a MS.
When the precursor was heated it lost some weight. The MS results show that some of the weight lost
is due to loss of water and some to burning of organic compound. The water loss happened in two
steps, both smooth and broad. The first peak appears after 14 – 19 min of heating which correspond to
a temperature about 85 to 100ºC. This temperature interval indicates that the water lost is from
desorption of adsorbed water. Wang et al. [3] found similar results when they activated the precursor in
a mixture containing butane, oxygen and helium. They found that the weight loss started at a
temperature of around 80ºC and from the MS trace they got two peaks for water.
The second peak appears at a temperature about 365 – 391ºC and appears therefore not in the
experiments, where the maximum temperature is only 260, although it is possible to see the beginning
of the peak (see Figure 5). It was found that the CO2 peak was detected later than the water peak by the
MS. The second peak can therefore be explained with the loss of crystal water during the
transformation of the precursor (VOHPO4·½H2O) instead of water from burning of organic
compounds.
Page 7 of 16
Exp 7
Exp 8
Exp 9
Exp 10
100
98
96
% wt
94
92
90
88
86
84
0
100
200
300
400
500
600
700
Tim e (m in)
Figure 6: Results from the TGA for experiment 7-10.
In all the experiment where the maximum temperature was 260ºC, it is seen that weight loss continues
when the temperature is kept constant and that results in a higher total weight loss at longer time (See
Table 3). The results from the MS show that the continued weight loss may results from loss of crystal
water or slowly burning of the organic compounds. When the maximum is 390ºC the weight stabilized
after a short period after the temperature was reached and then it was constant. At 480ºC it was seen
that weight gain was obtained when the gas was shift to air and even after 10 h it had not stabilized.
The weight gain was possibly caused by absorption of oxygen (See Figure 6).
In the experiment where only argon was used, the weight didn’t stabilize and a continued weight loss
was seen. This could be explained with a continued burning of organic carbon or loss of water. In this
experiment the highest total weight loss was obtained (See Table 3).
Table 3: The total weight loss in experiment 1–10.
1h
4h
10 h
10 h in Ar
260ºC
4.5%
5.3%
6.3%
390ºC
12.8%
12.9%
12.2%
460ºC
13.2%
13.1%
12.8%
15.0%
As in the tests the differentiated weight loss plot vs. time showed two peaks in the experiment at
temperature 390ºC and 460ºC, but only one in the experiments at 260ºC (See Figure 7). In all cases the
first peak had a maximum at about 84–100ºC and with the results from the MS, the weight loss can be
related to desorption of adsorbed water.
Page 8 of 16
The second peak was found about
Exp 4
Exp 5
Exp 6
371–376ºC, which is consistent with
the second peak in the MS trace for
possible to see a shoulder on the second
peak or third peak on the plot of the
d(%wt)/dt
water. In some of the experiment it is
differentiated weight loss vs. time. This
weight loss fits with the CO2 peak in
time and the weight loss can then be
0
50
explained with burning of organic
100
150
Tim e (m in)
Figure 7: The differentiated weight loss for experiment 4, 5 and 6.
7?????
compounds.
The colour of the precursor was dusty blue green, and after the experiments the colour had changed to
black except in experiment 6 and 9. In experiment 9 the colour was bright yellow green (See Figure 8)
and in experiment 6 the sample was not black as the others, but had a dark green grey colour. This
colour change and the weight gain indicate a change in oxidation number from 4+ to 5+.
a.
b.
c.
Figure 8: a. The sample from Exp. 9, b. The sample from Exp. 8 and c. The precursor.
Cheng et al. [23] has studied the effect of calcination environment on performance of the VPO catalyst.
It was found that calcination in air at 450ºC results in formation of a VOPO4 phase (V5+ species) and
when nitrogen was used the result was formation of the crystalline (VO)2P2O7 (V4+ species).
O’Mahony et al. [68] found the same results during their study of phase development during thermal
treatment of the precursor. That confirms the indication of at change in the oxidation number which is
seen by the change in colour.
Page 9 of 16
Conclusion
Because oxygen was presence during the heating, the organic compound burned and formed CO2, and
maybe CO but it was not possible to be detected by the MS because of the nitrogen. The result from the
MS showed two water peaks just as Wang et al. [3] found. The first peak may result from desorption of
adsorbed water and the second peak from loss of crystal water and burning of the organic compounds.
The TGA showed that a weight loss between 4.5% and 15% was obtained in the experiments. The
differentiated weight loss plots shows that the weight loss happened in one to three steps depending on
the temperature used. At 260ºC a weight loss was seen during the heating followed by a slow loss of
weight. At 390ºC a weight loss between 12.2 and 12.9% was obtained. The weight loss was only during
the heating, at 390ºC the weight was constant. At 460ºC a weight gain after the heating period was
observed, which was properly due to a change in the valence of vanadium in the catalyst.
Page 10 of 16
Future work
On the basis of the results it is found that a greater/larger knowledge about the precursor is needful. It
would be of great interest to know the total amount of organic carbon in the precursor together with the
overall composition. With that it would be possible to calculate a theoretically maximum weight loss.
In this series of experiments there is only performed one measurement in each experiment and it will be
a good idea to test the quality of reproduction.
Further experiments require some test of the TGA and MS to find the minimum level of nitrogen and
oxygen presence when only argon is used and how to get that level as fast as possible. Will a higher
flow of argon bring the level of Nitrogen and oxygen to the minimum faster?
In the oxygen and nitrogen free atmosphere it will be interesting to find the maximum weight loss at
the used temperatures (260, 390 and 480ºC).
In an atmosphere containing oxygen and argon (and where the level of nitrogen is as low as possible)
some of the experiment should be done again. From these two set of experiment the presence and shape
of CO and CO2 peaks should be studied.
It could also be interesting to calibrate the MS so it will be possible to find the concentration of CO2,
CO and O2.
It was fund that the sample from the TGA was too small to be analysed by XRD and SA, which would
had been a great help in the understanding of the calcination process. A larger sample could be made
by using a small reactor and run it of the same condition. By leading the off-gas through a trap it is
possible to study the acid evolution to see which and how much there is form. This is unfortunately not
possible with the TGA since the reaction chamber is open to the atmosphere and it is therefore not
possible to create the elevated pressure needed.
The oxidation number of vanadium could be of great value. It would be possible to receive the
oxidation number from the XRD if the crystal structure is identified. Guliants et al. [69] have published
a table with data about XRD and Raman peaks of some reference VPO phases at room temperature.
Another way to look at the oxidation number of vanadium in the catalyst is to calculate the average
value of vanadium valence. That is done by potentiometric titration [3], where the amount of V3+, V4+
and V5+ is identified. It will properly confirm a change in oxidation number in experiment 6 and 9 as
the colour change indicates. Further more the influence of temperature, amount of oxygen and water on
the rate of the change of Vox can be investigate.
Page 11 of 16
Page 12 of 16
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