NOTE
321
ityof71% at 10% conversion wasobtained (12). In the pulse
reactor, the selectivity to isobutylene was much higher. For
the first two pulses, the selectivity to isobutylene was 90%
and then approached 100% for the subsequent pulses. CO2
is the only by-product at this temperature. The average car-
bon balance after six pulses was 97% . The results are sum-
marized in Table 1.
The product distribution often depends on the nature
of reactant available in the reaction media. Since molecu-
lar oxygen in the gas phase is a distinctly different oxidant
from the oxygen species on the catalyst surface, its direct
reaction with hydrocarbon could be expected to give a dif-
ferent product distribution from that obtained only by sur-
face oxygen species. Thus any differences between redox
and cofeed modes of operation must firstly arise from re-
actions of hydrocarbon with either gas phase oxygen or the
oxidized surface. Normally, the redox mode reactor offers
higher selectivity. For example, the selective oxidation of
n-butane to maleic anhydride is carried out in a fixed-bed
reactor with a typical selectivity of 60% . Contractor et al.
FIG. 1. Catalyst surface area as a function of chromium loading.
the surface area of resulting catalysts varies between 20–
2
3
00 m /g depending on the loading level as shown in Fig. 1.
Although the chromium oxide in the catalyst exists in
(
14) found that up to 90% selectivity can be achieved by
both III and VI oxidation states, supported chromium oxide
is hereafter denoted as Cr2O3/La2(CO3)3.
using a circulating reactor.
The catalytic activity decreased by about 54% over the
first six pulses but the selectivity remained unchanged.
However, the catalyst activity, expressed as the amount of
isobutane reacted per gram of catalyst, and selectivity can
be restored by exposure to two pulses of oxygen at the re-
action temperature. This experiment is illustrated in Fig. 2.
In a redox mode of operation, the regeneration step often
requires a higher temperature. The reproducibility of this
catalyst activity after exposure to oxygen at the reaction
temperature indicates that the surface oxygen exchange on
the catalyst is very rapid.
The performance of catalysts has been assessed using a
pulse mode reactor with ancillary on-line gas chromatogra-
phy analysis. Gas flow rates were controlled by mass flow
controllersand monitored byPorter flowmeters. A six-port,
two-way air-actuated switching valve, controlled by a Valco
DVI unit and fitted with a 100-�l sample loop, allows ei-
ther pure carrier gas, isobutane, or oxygen to flow through
the reactor. A knockout trap immediately downstream of
the reactor removed the less volatile hydrocarbon products
(
if any) and water for off-line analysis. The remaining gas
phase product stream underwent on-line analysis using a
Varian 3700 gas chromatograph fitted with three columns:
Under the redox mode operation, oxidative dehydro-
genation of isobutane proceeded selectively. The average
(
i) a 12-in. VZ-7 column which provided analysis of CO2
�
and hydrocarbons from C1–C4 and connected to a ther-
mal conductivity detector; (ii) a 12-in. HayeSep N column,
also connected to a TCD to separate CO and CO2, and
selectivity over six pulses approached 100% at 220 C and
then decreased slightly as the temperature was increased.
Figure 3 shows the average selectivity after six pulses as
(
1
(
iii) a 7-in. VZ-10 column, connected to a FID to separate
-butylene and isobutylene, if necessary. The catalyst
100 mg) was prepared by a repeated process of pelletiz-
TABLE 1
ing and grinding to 60–80 mesh size before loading into the
middle of the reactor. A silica tube was inserted into the
bottom half of the reactor to eliminate possible gas phase
reactions. Normally, the catalyst was pretreated in situ with
He for 1 h at reaction temperature, but other pretreatments
with different conditions were possible.
�
Pulse Reactor Test for 25 mol% Cr2O3/La2(CO3)3 at 230 C
Isobutylene produceda
(�molisobutylene/g cat.)
Surface oxygenb
Pulse
(�mol/g)
1
2
3
4
5
6
2.45
2.00
1.56
1.39
1.17
1.13
1.22
1.00
0.78
0.69
0.58
0.56
The reaction was carried out at 150 kPa and at tempera-
�
tures in the range 200–500 C. Pure isobutane was injected
into the reactor via a 100-�l sample loop, giving a pulse size
of 4.25 �mol.
A catalyst consisting of 25 mol% Cr2O3 supported on
La2(CO3)3 was first selected for testing. This catalyst was
previously tested in a fixed-bed reactor. At 230 C a selectiv-
�
Note. At 230 C, after six pulses, isobutylene yield was 0.54 g/kg catalyst.
a
3
�
Calculated using disobutylene = 2.39 kg/m (gas density at 21 C).
�
b
Oxygen involved, calculated as O2.