Oxidative dehydrogenation over promoted chromia catalysts at short contact times

Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.

779

Oxidative Dehydrogenation over Promoted Chromia Catalysts at Short Contact Times D. W. Flick and M. C. Huff Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA The oxidative dehydrogenation of ethane and propane at millisecond contact times was studied using Cr203 and Pt coated ceramic foam monoliths. The supported Cr203 catalyst was able to achieve a higher selectivity to C2H4 at higher conversion of the hydrocarbon feed than the Pt coated monolith. I. I N T R O D U C T I O N The selective dehydrogenation of alkanes still remains a formidable obstacle in the wider use of lower alkanes as feedstocks for large industrial processes. Currently, olefins are used as important chemical intermediates for a large number of industrial processes. Thermal dehydrogenation of light alkanes to olefins is thermodynamically favorable at high gas temperatures (800-900~ but often leads to high yields of smaller hydrocarbons and coke [ 1]. In the industrial cracking process, steam is added to retard the deposition of coke on the walls of the reactor tube that eventually require the periodic shut-down of the process for decoking [ 1]. As a result, there has been much interest recently in trying to find a catalytic alternative to the current industrial steam cracking process. The catalytic oxidative dehydrogenation of hydrocarbons offers a promising alternative to thermal pyrolysis and catalytic dehydrogenation. In this study, we investigate C2H4 and C3H6 production by catalytic partial oxidation of C2H6 and C3H8 over chromium oxides supported on cx-A1203 and Mg-stabilized ZrO2 foam monoliths at millisecond contact times. In addition to identifying optimum feed conditions, the study examines the effect of the support on the activity of the metal oxide catalysts. The performance of these metal oxide catalysts is compared to the results from oxidative dehydrogenation over Pt coated monoliths which have been examined for the oxidative dehydrogenation of C2-C6 hydrocarbons [2-11 ].

2. EXPERIMENTAL 2.1 Reactor Configuration The reactor is essentially identical to that previously described for the production of syngas [ 12] and oxidative dehydrogenation of light alkanes [2]. The reactor consists of a quartz tube with an inner diameter of 20 mm. The catalyst is sealed in the tube with high temperature silica-alumina felt which prevents the bypass of gases around the catalyst. To reduce the radiation heat loss in the axial and radial directions and to better approximate adiabatic operation, inert foam monoliths are placed in front and behind the catalyst as heat shields, and the reaction zone is externally insulated. The reaction temperature was measured

780 by type K (chromel/alumel) thermocouples placed at the center of the reactor tube between the catalyst and the heat shield. The gas mixture for the reactor consists of C2H6 and 02 with N2 as the diluent, and are controlled by electronic mass flow controllers. The level of dilution ranged from 20 to 50%. The total feed flow rate to the reactor ranged from 1 to 3 SLPM which corresponds to an approximate contact time of < 10 milliseconds for the monolith catalyst. For all the experiments, the reactor pressure is maintained at 1.2 atm (18 psi). The autothermal reaction takes place over the catalyst around 900~ and a sample of the product gases is fed to a HP 6890 Gas Chromatograph (GC) through heated stainless steel lines. While the reaction operates autothermally at steady state, an external heat source is necessary to initially ignite the reaction. Over the Cr203 catalyst, the reaction ignites at approximately 350~ which is significantly higher than the 220~ preheat needed to ignite that reaction over a Pt coated monolith catalyst under the same conditions. After ignition, the external heat source is removed, the composition is adjusted to the desired value and steady state is established (<10 min.) before analysis of the reaction products by the GC where the N2 was used as an internal calibration standard. The atom balances, carbon and hydrogen, closed to within _+10%.

2.2 Catalyst Preparation In this study, the ceramic foam monoliths from Vesuvius Hi-Tech Ceramics with 45 pores per linear inch (ppi) are impregnated with a 0.25 M aqueous solution of Cr(NO3)3, and then calcined in air at 600~ for at least four hours. After calcination, the Cr203 coated monoliths are bright green in color. Several monoliths were further reduced in H2 at 600~ but the additional reduction step did not have any effect upon the color or reactivity of the catalyst. The chromium oxide was supported on monoliths made from 0;-A1203 (92%, A1203, 8% SIO2) and Mg-stabilized ZrO2. This process results in loadings of approximately 5-6 wt. % Cr203 on the 0:-A1203 monolith and 3 wt. % Cr203 on the ZrO2 monolith. Lower weight loadings are obtained by using a more dilute solution of Cr(NO3)3, and higher weight loadings could be achieved by repeating the process of impregnation and calcination. Pt was deposited on the monoliths similarly using a saturated solution of H2PtC16 [2]. The monoliths used measured 10 mm thick and between 18-20 mm in diameter. 3. RESULTS

3.1 Catalyst Screening The oxidative dehydrogenation of ethane was examined over a variety of catalysts supported on ceramic foam monoliths at a flowrate of 2 standard liters per minute (SLPM) which corresponds to a catalyst contact time of approximately 10 ms. The catalysts studied included 10 wt. % Cr203/t~-A1203, 9 wt. % Cr203/ZrO2, 3 wt. % Pt/t~-A1203, and Pt (0.05 wt. %) modified 10 wt. % Cr203/tx-A1203 catalysts. The 3 wt. % Pt/ct-A1203 and 9 wt. % Cr203/ZrO2 catalysts yield significantly higher selectivity to C2H4 than either the Cr203/(tA1203 or Pt modified Cr203/o~-A1203 catalysts. In general, the Cr203 catalysts show a slightly lower selectivity to CO than the Pt catalyst with somewhat more CO2 produced. The CO and CO2 selectivity for the Pt-modified Cr203 catalyst falls direct between the two single component catalysts.

781 Figure 1 illustrates the reaction results for the chromium oxide catalyst supported on a ZrO2 monolith (solid line) and the 3 wt. % Pt-coated t~-A1203 monolith (dashed line) for the oxidative dehydrogenation of ethane as a function of C2H6/O2 ratio in the feed with 20% N2 dilution at a flowrate of 2 SLPM. The catalysts show similar product selectivity trends with increasing ethane concentration in the feed. The C2H6 conversion and C2H4 yield is higher over the 9 wt. % Cr203/ZrO2 catalyst than over the Pt catalyst; the difference increases with increasing ratios of C2H6/O2 in the feed. At C2H6/O2 ratios of 1.2 and 1.5, the C2H4 selectivity for the two catalysts is essentially the same. But at the higher C2H6/O2 ratios, the selectivity to C2H4 is higher over the Cr203 catalyst at -70% compared to <65% over the Pt catalyst. Additionally, the CO selectivity is lower for the chromia-zirconia catalyst; at the higher C2H6/O2 ratios the CO selectivity is approximately half of the CO selectivity for the Pt catalyst with the selectivity to CO2 making up the difference. Even though the Cr203/ZrO2 has a higher C2H6 conversion and higher selectivity to CO2, the lower selectivity to H20 causes the reaction temperature of the Cr203/ZrO2 catalyst to be lower than the temperature of the Pt catalyst by approximately 100~ 3.2 Catalyst Lifetime

In addition to product selectivities, the lifetimes of the Cr203 and Pt catalysts under reaction conditions also differ. For the Pt-coated monolith, reaction has been shown to be

8O 7O E~"

60

17)

5o

~

("

.-

'

'

'

I

'

'

'

. (a)

I

'

'

'

I

_

'

'

'

I

'

'

'

I

'

'

'

I

'

'

'

1 4 ~ '

~~-

40

..Q

-~ 30 o ~o 20 10 0

'

I

'

'

'

I

'

_

lOO

,

,

I

,

1.4

,

,

I

,

,

,

1.6

I

,

,

,

1.8

I

,

,

,

2

I

,

,

,

2.2

.o ..~ 80 -(") ~ 70

'

'

'

I

'

'

'

I

'

'

' (It) i

'

'

8 ..,,4

I

,

,

,

2.4

2

2.6

' 1.2

1.4

1.6

'

I

'

'

'

I

'

---~-tK,~.CH

"

-Q~"

'

1.8

2

2.2

2.4

2.6

Fuel/Oxygen

. . I • ' • • • ' i (C)

90

I

~

Fuel/Oxygen ' ~ "

'

4 ,

1.2

~-"

'

~

'

I

'

'

'

I

'

'

Conversion

1 0 5 0 1(,-~_\ ~, ' ,, ,, ,, , , ,, , , ,, , , ,,,

'

_

~ " ~ ~" ' ~ ' ~ I N ~ _

,.-- ~, 60 ~ 50

i

.~ G" 1000 "~

~

950

o

.~

900

,, ,,

d)

r..

~ 850

=*'~

no

~m

30

20 1.2

C1-14 Yield ,,,

Jl

1.4

;n

ml,

1.6

an

I,

1.8

" " " 11-,..,..1 n;

I

;

2

Fuel/Oxygen

,=

In

2.2

nn

I,

2.4

t--

800 750

nn

2.6

1.2

1.4

1.6

1.8 2 2.2 Fuel/Oxygen

' ' ' 2.4 2.6

Fig. 1" Carbon atom selectivity (a,b), C2H6 conversion (c), C2H, yield (c), and catalyst temperature (d) ver a Cr203/ZrO 2 monolith (solid line) and Pt/tx-A1203, monolith (dashed line), as a function of C2HJO 2 ratio in the feed with 20% N 2 dilution with a total feed flow rate of 2 SLPM at a pressure of 1.2 atm.

782 Table 1 catalyst lifetime (hrs.) as a function of C21-LJO~ratio in the feed ~th 20% I~ dilution with a total feed flow rate of 2 SLRVI in an autothen~ reactor at a pressure of 1.2 arm. The catalyst lifetime is the length of time the reaction remainsignited over the catalyst at each specific flow coition before the reaction

exUngashes. L~eti~ (Hr.) ii

3%

FuellO~

9%

2%

10%

1.50%

O. 0 5 % Pt -

Pt/~Al20a Cr20ETJ'02 Cr20E-/_r02 Cr2Oa/~AI20a Cr20J~AI20a 10% 0"20a/~AI20a

1.2

>5.0

**

>2_0 **

>2_0 * *

>4.0

1.5

> 5.0 **

> 2.0 **

> 2_0 **

1.6

1.8

> 5.0

> 6.0 **

0.1

2_0

> 5.0 **

> 2_0 **

2_2

> 5.0 **

> 2_0 **

-. -.

2_4

>5.0

>2_0 **

-.

(**) Catalyst

**

**

**

0.1

> 1.0 **

<0.1

> 7.0 **

O.1

.

.

--

.

.

.

.

.

. . .

.

9.0

. . .

did not showany signsof deactivation

sustained for very long periods with no apparent deactivation or product selectivity changes for a wide range of C2H6/O2 ratios. The supported Cr203 catalysts in this study, however, have been shown to have a limited lifetime which is defined as the length of time the reaction remains ignited over the catalyst at specific flow conditions before the reaction extinguishes. In our investigation, the lifetime of the catalysts was shown to depend on the C2H6/O2 ratio, the support material, the weight loading of the catalyst, the flowrate, the N2 dilution, and the preheat of the feed. The lifetime of the various catalysts are shown in Table 1 as a function of C2H6/O2 ratio with 20% N2 dilution at a flowrate of 2 SLPM. The Pt/ff-A1203 and 9 wt. % Cr203/ZrO2 catalysts are much more stable than the other catalyst compositions examined. For example, the 10 wt. % Cr203/A1203 catalyst at a C2H6/O2 ratio of 1.2 sustained reaction for very long periods of time with no signs of deactivation. However as shown in Table 1, at higher C2H6/O= ratios, the reaction is sustained for < 1.6 hours. Before extinction of the reaction, the product selectivities and reactant conversions are fairly constant over the entire time period. In contrast to the Cr203/ff-AlaO3 catalyst, the reaction lifetime for 9 wt. % Cr203/ZrO2 catalyst showed no deactivation for C2H6/O2 ratios up to 2.4. For example at a C2H6/O2 ratio of 1.8, the 9 wt. % Cr203/ZrO2 monolith catalyst did not show any deactivation for over 5 hours of continuous operation, while the reaction quickly extinguished under these condition for lower Cr203 loadings on ZrO2 and on all Cr203/ff-A1203 catalysts. In addition to the support material, it can also be seen in Table 1 that the Cr203 loading of the monoliths can significantly affect the lifetime of the catalysts.

3.3 Limited Catalyst Lifetime and Reaction Front As described above, in some cases the supported Cr203 catalyst extinguished rapidly. When the catalyst extinguished, the process could be followed by observing the temperatures at the front and back faces of the monolith. Initially, the temperature measured at the front of the monolith is higher than the back temperature. With increasing reaction time, the front temperature would initially fall rapidly from 900~ to around 600~ and then the temperature

783 usually less than 20~ Finally, the back temperature quickly falls as the reaction extinguishes. Since the temperatures can be used to follow the deactivation of the catalyst, the reaction was stopped at various times and the catalyst was removed and examined. For all cases, the front face of the catalyst was green (Cr203), while the rear section was black (coke and possibly CrO). There was a very sharp line between the green and black sections. The longer the reaction time the further the green section proceeded down the length of the catalyst. After the catalyst extinguished, the monolith was completely green. At this point, the monolith could be reignited by applying external heating, and the catalyst would go through the same cycle of slow deactivation and finally extinction. Thus, the reaction front on the catalyst can be located by observing the line between the green and black sections. It is widely known that the oxidation state of chromium oxide can be changed fairly easily and rapidly. Therefore, the transition on the catalyst between the green section and black section marks the transition in the chromium oxide catalyst between an oxidizing environment and a reducing environment. The supported Cr203 catalyst acts essentially as two catalysts in the two reaction environments. In the oxidizing environment near the front face of the catalyst, the supported Cr203 acts as a relatively active and selective oxidative dehydrogenation catalyst. After the majority of the oxygen has been consumed at the front end of the catalyst, the supported Cr203 catalyst, as well described in the literature, acts as a highly active and selective dehydrogenation catalyst by utilizing the heat generated by the highly exothermic oxidation reactions at the front of the monolith. Therefore, the

60 50 ~Eo C'o

-

i')' '' '''''

''

',''

50

'-I

40

40

1

o=~ 20

-Q

0.8

(b) .~

~ ' - " 30

30

0

~',~ -,,co

Q)

f

0

1

1.2

1.4

1.6

1.8

2

2.2

i_..----~I

rl

0.8

2.4

i

I

i

i

1

,

I

,

1.2

Fuel/Oxygen ._.~"

100

1000

80 "~

~" 950

,,

I,

,

1.4

,

I

i

1.6

i

,

I

Ill

1.8

Fuel/Oxygen

,,.,,,l,,,

,,,, .,,,l,,,

l

I

I

I

2

I

I

1 11

2.2

2.4

, , , , l,,,:~

I ~ .

(d)

1

~" 900

60

-~

c o

~

850

'- '-,,,...

800 ~.

20

I- 750

0

~-

0

7 0 0

0.8

1

1.2

1.4

1.6

1.8

Fuel/Oxygen

2

2.2 2.4

, , I

0.8

1

, ,

, I

, , ,

1.2

I ,

1.4

, ,

I i ,

1.6

, I

, , ,

1.8

I ,

2

, ,

I i,

I

2.2 2.4

Fuel/Oxygen

Fig. 2: Carbon atom selectivity (a,b), propane conversion (c), and catalyst temperature (d) over a Cr203/ZrO2monolith (solid line) and Pt/ct-A1203, monolith (dashed line) as a function of C3HJO2ratio in the feed with 50% N 2dilution with a total feed flow rate of 2 SLPM at a pressure of 1.2 atm.

784 unreacted C2H6 in the reducing environment is dehydrogenated by the reduced chromium oxide surface. Thus at the higher C2H6/O2 ratios where there is more unreacted C2H6, the Cr203 monolith should and does show higher C2H6 conversion and C2H4 selectivity than the Pt monolith alone.

3.4 Oxidative Dehydrogenation of Propane The oxidative dehydrogenation of propane was examined over Cr203 and Pt supported on ceramic foam monoliths at a flowrate of 2 SLPM. Figure 2 illustrates the reaction results for the Cr203 on a ZrO2 monolith (solid line) and the 3 wt. % Pt-coated ~-A1203 monolith (dashed line) as a function of the C3H8 to 02 ratio with 50% N2 dilution. The product selectivities and reactant conversion, as in the oxidative dehydrogenation of ethane, follow the same trends with changes in the fuel-to-oxygen ratio. The C2H4, total olefin, and C3H8 conversion are higher over the Cr203 catalyst than over the Pt catalyst. The selectivity to C3H6, however, is higher over the Pt catalyst. Similar to the ethane case, the CO selectivity is lower and the CO2 selectivity is higher for the Cr203 catalyst. Additionally at the higher C3H8/O2 ratios, the Cr203 catalyst has a slightly lower reaction temperature than the Pt catalyst. 4. CONCLUSION It has been found that supported Cr203 is a good oxidative dehydrogenation catalyst. With a Cr203 catalyst supported on a ceramic foam monolith at contact times on the order of milliseconds, ethylene is produced with high conversion of the hydrocarbon feed and complete consumption of oxygen at atmospheric pressure. The Cr203/ZrO2 catalyst had the highest C2H4 selectivity along with the lowest CO selectivity of all the catalysts examined. We propose that the improved C2H4 selectivity and fuel conversion results from the catalytic nature of the supported Cr203. In the front section of the reactor where 02 is present, supported Cr2Os is an active oxidative dehydrogenation catalyst. After the 02 has been consumed, the supported Cr203, which is a well-known active and highly selective dehydrogenation catalyst, reacts with the unreacted fuel by utilizing the heat generated by the highly exothermic oxidation reactions. Under some operating conditions, the Cr203 catalysts have a limited active lifetime, unlike the Pt/c~-A1203 catalyst which can operate for very long periods of time with no deactivation. The lifetime of the supported Cr203 catalysts depends primarily on the feed conditions, support material, and catalyst loading.

References (1) (2)

(3) (4)

(5) (6) (7)

(8) (9)

(10) (11) (12)

Pyrolysis: Theory and Industrial Practice; Albright, L. F.; Crynes, B. L.; Corcoran, W. H., Eds.; Academic Press: New York, 1983. Flick, D. W.; Huff, M. C., Catal Lett 47 (1997) 91-97. Flick, D. W.; Huff, M. C., J. Catal. 178 (1998) 315-327. Dietz III, A. G.; Carlsson, A. F.; Schmidt, L. D., J. Catal. 176 (1998) 459-473. Huff, M. C.; Schmidt, L. D., J. Catal. 149 (1994) 127-141. Astbury, C. J.; Griffiths, D. C.; et.al., US Patent 5382741 (1995), to British Petroleum Company p.l.c. Huff, M.; Schmidt, L. D., J. Phys. Chem. 97 (1993) 11815-11822. Heck, R. M.; Flanagan, P., US Patent 4844837 (1989), to Englehard Corporation. Font Freide, J. J.; et.al., US Patent 5105052 (1992), to British Petroleum Company p.l.c. Golunski, S. E.; Hayes, J. W., US Patent 5593935 (1997), to Johnson Matthey PLC. Witt, P. M.; Schmidt, L. D., J. Catal. 163 (1996) 465-475. Hickman, D. A.; Haupfear, E. A.; Schmidt, L. D., Catal Lett 17 (1993) 223-237.