Oxidative dehydrogenation of ethane over a perovskite-based monolithic reactor

Journal of Catalysis 209, 51–61 (2002)

doi:10.1006/jcat.2002.3636

Oxidative Dehydrogenation of Ethane over a Perovskite-Based

Monolithic Reactor

∗

,1

∗

Francesco Dons`ı, Raffaele Pirone,† and Gennaro Russo

∗

Dipartimento di Ingegneria Chimica, Universita` “Federico II” di Napoli, P. le Tecchio 80, 80125 Napoli, Italy;

and †Istituto di Ricerche sulla Combustione, CNR, P. le Tecchio 80, 80125 Napoli, Italy

Received September 17, 2001; revised March 15, 2002; accepted April 11, 2002

In recent years, a renewed interest towards partial oxi-

dation processes of light alkanes has been sparked by the

results of the group of Prof. Lanny Schmidt at the Uni-

versity of Minnesota. Since 1993, they have demonstrated

that oleﬁns can be efﬁciently produced by the autother-

mal oxidative dehydrogenation of ethane over Pt contain-

ing structured reactors at short contact times (milliseconds)

and temperatures in the range of 900–1000^◦C (5). Under

such reaction conditions, it was shown that feeding a mix-

ture C₂H₆/O₂in a molar ratio slightly lower than 2 (namely,

the stoichiometric value for ethane ODH) makes the ethy-

lene yield high enough to be compared with the existing

cracking processes.

Thechoiceofplatinum-basedcatalystsforthisprocessre-

sultedfromtheinvestigationsofSchmidtetal. (6), whocom-

pared the activity of different noble metals in short-contact-

time reactors for partial oxidation processes, reporting Pt as

the best catalyst for ethane ODH and Rh as the most suit-

able in partial oxidation to syngas. The other noble metals

tested for CH₄partial oxidation did not show promising

performances or enough stability. Pd, for instance, quickly

deactivated due to coke formation (7). Moreover, doping

Pt with tin or copper resulted in improved performance for

ethane ODH, with a signiﬁcant increase in ethylene yield

(8), which was even greater as a result of H₂addition (9).

The nature of these results remains disputed. Until re-

cently, it has been reported that processes occurring in this

kind of reactor are purely heterogeneous (5, 10, 11) and can

be regulated by the appropriate choice of the active compo-

nent (12), support morphology (11), and reaction mixture

composition (5). This seems to be true only at moderate

temperatures, while above 800^◦C, it has been shown, both

experimentally (13–17) and theoretically (18–20), that the

kinetics of homogeneous reaction paths is not negligible,

even for millisecond residence time scales.

The oxidative dehydrogenation (ODH) of ethane has been in-

vestigated in a short-contact-time reactor consisting of a LaMnO₃-

based monolithic catalyst with a honeycomb morphology. Using

an ethane/air mixture with a C₂H₆/O₂ratio = 1.5 and a preheat

temperature ranging from 250 to 400^◦C results in a 55% ethy-

lene yield, a value even higher than those reported in the same

experimental conditions over Pt-based catalysts. By investigating

the effect of the experimental conditions, we found that the major

role of increasing the feed ﬂow rate and decreasing the C₂H₆/O₂

ratio is to raise the degree of adiabaticity of the reactor and, con-

sequently, ethane conversion and temperature. The selectivity to

ethylene also seems to increase with increasing temperature, but

only up to about 950^◦C. At higher temperatures, further degra-

dation of ethylene to C₂H₂and CH₄occurs. We also found that,

unlike Pt monoliths, the perovskite catalyst is intrinsically active in

c

ODH.

ꢀ 2002 Elsevier Science (USA)

Key Words: ethane; ODH; ethylene; short contact time; monolith;

oxidative dehydrogenation; autothermal reactor.

INTRODUCTION

The abundant availability of natural gas stimulated the

study of novel and more efﬁcient processes for its exploita-

tion. Although combustion processes for thermal energy

production represent the most practised way of utilysing

natural gas, the chemical transformation of small alkanes

into more valuable or transportable compounds is also

a matter of intensive and interesting research (1–3). Ex-

tensive work has been devoted to studying oxidative pro-

cesses for ethylene production, owing to the potential ad-

vantages of exothermic and thermodynamically unlimited

reactions, coupled with the possible reduction of coke de-

position in the furnaces. Speciﬁcally, the catalytic oxidative

coupling of methane (OCM) and the oxidative dehydro-

genation of ethane (ODH) have been investigated thor-

oughly, but among all catalysts considered, none seems to

exhibit promising performance for large-scale applications

in conventional reactors (4).

The alternative possibility to a purely heteroge-

neous mechanism of ethylene production is a hetero-

homogeneous reaction path. As proposed in recent works

(13), the catalyst could be assumed to act mostly as an

igniting agent, active for total oxidation reactions, while

ethylene-forming reactions mainly occur in the gas phase.

¹To whom correspondence should be addressed. Fax: +39 0815936936.

E-mail: pirone@irc.na.cnr.it.

51

0021-9517/02 $35.00

c

ꢀ 2002 Elsevier Science (USA)

`

DONSI, PIRONE, AND RUSSO

52

In the same way, Bodke et al. (9) sketched a short-contact- of methane over Pt-based monoliths (23). Since then, foams

time reactor as a two-zone reactor where in the ﬁrst zone have been mainly investigated for ODH reactions (5, 9, 21).

oxygen depletion occurs through C₂H₆oxidation to CO_xSadykov et al. (24) used honeycombed monoliths in oxida-

and H₂O with consequent temperature increase, while in tive dehydrogenation of propane to reduce secondary cata-

the second zone, where O₂is absent, the endothermic de- lytic reactions of propylene through the straight channels

hydrogenation of ethane to ethylene and H₂takes place. of the monolithic support. Only limited interest has been

Moreover, Bodke et al. (9) also reported that the use of hy- devoted to noble metal gauzes (13), because of the large Pt

drogen co-feeding allows a minor ethane consumption in consumption due to volatilisation at the high temperature

the ﬁrst zone of the reactor, with H₂being oxidised more of operation of gauze reactors. FeCr alloy elements packed

easily than ethane over the Pt–Sn surface, and consequently in a quartz tube were used as an alternative to foams, be-

C₂H₆dehydrogenates in the second zone with very high cause of their high void fraction and large thermal conduc-

selectivity (>80%) and yield (about 60%) to ethylene. De- tivity (15, 19). However, there is no assurance that foams

tailed heterogeneous and homogeneous chemical kinetic are more useful than honeycomb-type monolith catalysts

mechanisms employed in the two-dimensional computa- in ethane ODH, as the conclusions of the work of Hickman

tional ﬂuid dynamics model developed by Zerkle et al. (20) and Schmidt (23) can be strictly applied only to the partial

conﬁrmed the main scheme of Bodke et al. (9) and showed oxidation of CH₄, in which the role of homogeneous reac-

that the relative contributions of heterogeneous and ho- tion paths is much less relevant, if not negligible, than in

mogenous reactions to the ethane overall consumption are the case of ethane. ODH is different from partial oxidation

strongly dependent on C₂H₆/O₂feed ratio. More speciﬁ- to syngas, because the latter is a process that is widely ac-

cally, they reported that the formation of ethylene must be cepted to occur to a larger extent on the catalytic surface

mainlyattributedtoheterogeneousdehydrogenationpaths, and is then more inﬂuenced by the larger geometric sur-

especially when H₂is added to the feed.

face of foam monoliths. As a consequence, we think that

The use of materials other than noble metals has been honeycombed monoliths for the application of the short-

considered by some authors (16, 21, 22). Flick and Huff contact-time concept in ethane ODH could be preferable,

proposed the use of Cr₂O₃-based foam monoliths in ethane as their straight channels could better preserve ethylene

ODH (21), showing that they exhibit very promising per- from further reacting in secondary paths.

formance, even better than Pt-based monoliths, but with

Finally, we have considered the study of non-noble-

a strongly limited catalyst lifetime, likely due to the well- metal-based catalysts in the oxidative dehydrogenation of

known deactivation processes occurring on transition metal ethane in short-contact-time reactors with a honeycomb-

oxides in the catalytic combustion of hydrocarbons (i.e., type morphology. In this ﬁeld, we have recently developed

under operating conditions characterised by high tempera- novel structured catalytic systems, active in hydrocarbon

tures, oxidising atmosphere, and the presence of water and deep oxidation processes as an alternative to noble metals

CO₂).

(25). Speciﬁcally, the activity of a LaMnO₃-based mono-

BaMnAl₁₁O₁₉hexa-aluminate, characterised by a high lithic catalyst has been proven to be stable in the catalytic

thermal stability, was investigated by Beretta and Forzatti combustion of methane under 100 h of operation in ignited

under autothermal conditions (22). In comparison with conditions at 1000^◦C (26). To test the applicability of such

Pt/α-Al₂O₃catalyst, hexa-aluminate showed better perfor- a catalyst in ethane ODH at short contact time, we have

mance, even though the higher preheat temperature re- performed reaction tests under a wide range of experimen-

quired caused coke deposition. However, the authors did tal conditions with the aim of both assessing the ultimate

conclude that the presence of any catalyst could be useful role of the catalyst properties in the reaction paths leading

in making the process autothermal but reduces the max- to oleﬁns and of evaluating the possibility of using a signiﬁ-

imum yield attainable in the homogenous phase by only cantly cheaper catalyst (compared to Pt-based ones) in the

heating the C₂H₆/O₂mixture. In particular, they clearly ev- development of a novel and more efﬁcient process of oleﬁn

idenced that Pt catalysts under controlled isothermal con- production.

ditions, namely, in the absence of any homogeneous contri-

bution, do not exhibit any intrinsic activity in ethane ODH

to ethylene, and in short-contact-time reactors the Pt cata-

EXPERIMENTAL

lysts are hence responsible of the higher amount of ethane

oxidised to CO_xcompared to a pure homogeneous process

Catalyst Preparation

(15, 22).

The preparation of perovskite-based honeycomb mono-

The morphology of the catalyst used for such processes liths via active phase deposition on cordierite is essentially

has also been under investigation. Foams have been thor- identical to that previously described (25). The ceramic sub-

oughly studied since 1992 by Hickman and Schmidt (23), strates (Corning) with a cell density of 400 cpsi (cells per

who found clear evidence that extruded honeycombed square inch) were washcoated with alumina by repeated

monoliths performed worse than foams in partial oxidation dipping in a slurry of ﬁnely grounded γ -Al₂O₃powder,

ETHANE ODH OVER PEROVSKITE-BASED MONOLITH

53

diluted nitric acid solution, and pseudobohemite. La₂O₃4 and 50 ms. Feed gas composition was varied with a

(5% wt) was added to the washcoat by impregnation, fol- C₂H₆/O₂ratio between 1 and 2. Product gases were fed

lowed by calcination at 800^◦C for 3 h. The LaMnO₃active to a CaCl₂trap to selectively remove water, prior to split-

phase (30% wt) was added to the monolith using the wet ting to an on-line gas chromatograph and to a Hartmann

impregnation method.

& Braunn Caldos analyser, which was employed to measure

H₂concentration.

Autothermal Reactor Conﬁguration

RESULTS

Catalytic activity measurements of ethane ODH at short

contact times were carried out by placing two blank

cordierite monoliths, 2 cm long, upstream and downstream

of the catalytic monolith, in order to limit as much as pos-

sible heat losses by radiation, and sealing all three with

quartz wool in a steel tube. The whole system was placed in

an electric furnace to preheat the gas and solid. The annular

gap between the steel tube and the furnace walls was com-

pletely ﬁlled with ceramic wool to limit as much as possible

heat losses from the laboratory-scale reactor under ignited

conditions.

The monolith employed in autothermal tests was char-

acterised by an open cross section of 7 × 7 channels and a

length of 40 mm. The central channel was blocked for wall

temperature measurement at four different locations (up-

stream cordierite monolith, inlet and outlet of the catalytic

monolith, and downstream cordierite monolith) using K-

type thermocouples (d = 0.5mm), two of which enter from

the top and two from the bottom of the reactor. As a con-

sequence, the number of working channels for the catalysts

was 48. A high value of ethane concentration was used in

such tests to achieve a large heat production rate from the

chemical reactions.

We carried out ODH tests under different experimen-

tal conditions in both autothermal and isothermal reactor

conﬁgurations, as described in the previous section. In au-

tothermal tests, the high temperature attained, together

with the presence of a large amount of water produced by

the reaction, generated conditions strongly favourable to

sintering phenomena. Moreover, the temperature spatial

gradients as well as the thermal shocks undergone by the

catalyst in start-on/shut-off operations were also relevant

and potentially capable of deactivating the catalyst. Nev-

ertheless, the same monolithic sample used in the whole

investigation resulted in very stable and repeatable exper-

iments for about 90 h of operation. Similar performances

of thermal stability properties of this catalyst were already

demonstrated in a previous investigation on methane cata-

lytic combustion (26).

Representative results of the experimental campaign are

reported here. In all the autothermal runs, the conversion of

O₂was complete in the steady state, independently of oper-

ating conditions, while the values of selectivities, yields, and

temperatures were signiﬁcantly variable. The maximum re-

actor temperature seems to be the key factor in determining

the catalyst performance. The higher the temperature of the

reactor, the higher the ethane conversion achieved. Never-

theless, the highest selectivity to ethylene seems not to be

reached at the highest temperature attained but lies in the

range of 850–950^◦C, as already reported by Flick and Huff

(27). Indeed, below this temperature range ethylene is not

signiﬁcantly produced, while above 900^◦C acetylene, CH₄,

and CO are favourably formed. The main parameters of the

process (ethane/oxygen ratio, total ﬂow rate, and preheat

temperature) had to be tuned to keep the temperature in

the optimal range.

Isothermal Reactor Conﬁguration

A smaller catalytic monolith, characterised by eight

working channels, was employed in isothermal tests. In con-

trast to the autothermal measurements, in this case quartz

wool was removed from the gap between the steel tube

and the furnace walls. To carry out a correct characteri-

sation of catalyst performance in ethane partial oxidation,

experimental conditions have been adjusted to exclude the

onset of gas-phase reactions. The system can be considered

isothermal for a mixture with an inlet molar fraction of

ethane as low as 0.5%. By reducing heat production on the

surface, the difference between catalyst and oven temper-

ature can be maintained to within a few degrees Celsius.

Flow Rate

The effect of space velocity was investigated using a

In this way gas-phase temperature can be kept to less than C₂H₆/O₂/N₂= 24/16/60 mixture (ethane in air) at a pre-

600^◦C, which was considered the threshold temperature for heating temperature of 400^◦C. Space velocity was changed

homogeneous reactions.

by varying feed ﬂow rate between 35 and 90 Nl/h (l/h mea-

Gas ﬂow into the reactors was controlled by three sured at 273 K and 1 atm), i.e., the contact time between

Brooks mass ﬂow controllers, respectively, for ethane, oxy- 77 and 200 ms · cm³/Ncm³. Under the actual operating con-

gen or air, and nitrogen. GHSV (Gas Hourly Space Ve- ditions this corresponds to an interval from 20 to 50 ms,

locity) was varied from 18,000 up to 480,000 h⁻¹as eval- evaluated at an average temperature of 900^◦C.

uated at standard conditions on the basis of monolith

Figure 1a shows that ethylene yield reaches a maximum

volume, corresponding to residence times ranging from value for a total ﬂow rate of about 70 Nl/h. Such a maximum

`

DONSI, PIRONE, AND RUSSO

54

Nevertheless, in the upstream radiation shield the highest

Total flow rate (Nl/h)

temperature was observed in the case of the lower ﬂow rate,

because of both the higher retro-diffusional effects and the

lower extent of cooling of the solid.

30 40 50 60 70 80 90

100

80

a.

O₂

Higher ﬂow rates lead to higher temperatures and ethane

conversions, because the system becomes more adiabatic,

as already shown in a previous investigation (28). In the

energy balance, indeed, the heat generation term is bound

to the total ﬂow rate and the amount of ethane converted,

while the heat dissipation term depends only on the ge-

ometry of the system and on the maximum temperature.

Around 1000^◦C, radiation becomes the major dissipation

route, and this feature leads also to a self-adjusting max-

imum temperature. With this in mind, it seems better to

correlate the observed variation of conversion and selec-

tivities with the temperature proﬁle established in the reac-

tor, rather than with the change in contact time. Actually,

the singular increase of reactant conversion with decreasing

contact time appears very reasonable, taking into account

the contemporary temperature increase.

C₂H₆

60

C₂H₄

40

20

0

b.

70

C₂H₄

60

20

CO₂

On the other hand, the selectivity to ethylene weakly de-

creases with increasing space velocity, mainly due to the ap-

pearance of other hydrocarbons, produced by C₂H₄degra-

dation favoured at higher temperature.

CO

10

CH₄

C₂H₂

Similar results were also reported on Pt-based foam

monoliths (5), where the increase of ethane overall con-

version and ethylene yield with increasing feed gas rate was

shown for contact times between 20 and 100 ms · cm³/Ncm³.

Such results were discussed by means of a completely het-

erogeneous reaction mechanism; the authors argued that

“the conversion increases at high ﬂow rates because short

contact times favour partial oxidation products.” More

speciﬁcally, they explained that “At long contact times, the

reactants remain on the catalyst surface long enough to pro-

duce complete combustion products, CO₂and H₂O. This

quickly consumes all of the O₂so oxidative dehydrogena-

tion is greatly reduced; thus some C₂H₆remains unreacted”

(5). However, the authors reported that the exit tempera-

ture increases with the ﬂow rate from 750 to 900^◦C, thus

supporting our assumption that reactor temperature plays

the major role in determining conversion and selectivites, as

revealed by the measurements of axial temperature proﬁle

established in the monolith (Fig. 2).

0

60

c.

HC

50

40

30

20

10

0

H₂O

H₂

30 40 50 60 70 80 90

Total flow rate (Nl/h)

FIG. 1. O₂and C₂H₆conversion and C₂H₄yield (a), C-atom selectiv-

ity (b), and H-atom (c) selectivity at varying total inlet ﬂow rate. Operating

conditions: C₂H₆/O₂/N₂= 24/16/60; T_preheat= 400^◦C.

occurs because ethane conversion increases (from 66 to

87%) at increasing ﬂow rate, while the selectivity to ethy-

lene is observed to decrease from 66 to 59% (Fig. 1b).

C₂H₆/O₂Ratio

The C₂H₆/O₂ratio plays a signiﬁcant role in determin-

Actually, while the production of CH₄and C₂H₂is more ing the reactor temperature and affecting the selectivity.

favoured at the highest velocities explored, the overall se- Figure 3 reports the behaviour of reactant conversion and

lectivity to hydrocarbons remains constant (Figs. 1b and product selectivity as functions of C₂H₆/O₂ratios and con-

1c), suggesting that these species are formed via in-series stant feed ﬂow rate. The experiments were carried out at

mechanisms of ethylene degradation.

constant ethane concentration (24%), enriching air with

In Fig. 2, the temperature proﬁles for the three highest oxygen at lower ratios and with nitrogen at higher ones.

ﬂow rates investigated are shown. At 90 Nl/h, the maximum The nominal contact time of the runs was 96 ms · cm³/Ncm³,

temperature was 1000^◦C, while at 72 Nl/h it was 50^◦C lower. which under real operating conditions corresponds to about

ETHANE ODH OVER PEROVSKITE-BASED MONOLITH

55

wards a slightly lower value (the optimal ratio C₂H₆/O₂was

equal to 1.5), probably due to the higher amount of CO₂

generally found among reaction products in our tests and

consequent higher O₂demand.

1000

900

800

700

600

Preheating Temperature and Transient Behaviour

The best performances in terms of ethylene yield have

been reached at a C₂H₆/O₂ratio of 1.5 and a ﬂow rate of

72 Nl/h. Under these conditions, we investigated the effect

of the preheat temperature. The experiments have been

carried out by repeatedly increasing the temperature of the

system by 20^◦C and waiting for the establishment of steady-

state conditions.

90 Nl/h

80 Nl/h

72 Nl/h

Radiation

shield

Radiation

shield

Catalytic monolith

0

10 20 30 40 50 60 70 80

Reactor Depth (mm)

C₂H₆/O₂

FIG. 2. Temperature proﬁles in the reactor at varying total inlet ﬂow

rate as a function of reactor depth. Catalytic monolith is placed between

20 and 60 mm. Radiation shield monoliths are placed upstream and down-

stream. Operating conditions: C₂H₆/O₂/N₂= 24/16/60; T_preheat= 400^◦C.

1.2

a.

1.4

1.6

O₂

1.8

2.0

2.2

100

80

60

40

20

0

C₂H₆

24 ms (by assuming a uniform gas temperature of 900^◦C;

see Fig. 2).

Ethane conversion was observed to decrease with in-

creasing C₂H₆/O₂ratio, while the selectivity to ethylene

was signiﬁcantly enhanced by the defective oxygen, grow-

ing from 50% at the lower ratio investigated up to 70% at

the higher one (Fig. 3b). Nevertheless, ethylene yield ex-

hibited a maximum (Fig. 3a), since conversion decreased in

the same conditions from 93 to 58%.

C₂H₄

70 b.

C₂H₄

60

Oxygen conversion was always complete, but product

distribution markedly changed. At C₂H₆/O₂= 1.3, larger

amounts of CO₂, CH₄, C₂H₂, and H₂were measured than

at higher ratios, while an increase of C₂H₄, CO, and H₂O

was observed on increasing the C₂H₆/O₂ratio to 2.

We reckon that the defective oxygen content plays an

important role in changing ethane conversion and ethylene

selectivity. Nevertheless, the formation of CO, CH₄, C₂H₂,

and H₂is more reliably bound to the reactor temperature,

which is higher at lower C₂H₆/O₂ratios due to the higher

exothermicity of the mixture.

50

CO₂

20

10

CO

CH₄

C₂H₂

c.

0

60

HC

50

40

30

20

10

0

Flick and Huff (27) pointed out that since the adiabatic

temperature is higher at lower C₂H₆/O₂ratios, this has a

strong inﬂuence on product distribution and above all on

productsformedinthegasphase, forinstanceacetylene, but

also CH₄, CO, and H₂, which are enhanced by high temper-

ature. Such ﬁndings are conﬁrmed by the results reported

over different active phases, such as Pt (5), Pt–Sn and Pt–

Cu (8), Pt–Cr₂O₃(21), and BaMnAl₁₁O₁₉(22), and also by

a detailed computational study (20). Accordingly to our re-

sults, over these catalysts it was reported that, at increasing

C₂H₆/O₂ratio, ethane conversion decreases while ethylene

selectivity increases, corresponding to a decrease in reac-

tor temperature. This gives rise to a maximum yield around

H₂O

H₂

1.2

1.4

1.6

1.8

2.0

2.2

C₂H₆/O₂

FIG. 3. O₂and C₂H₆conversion and C₂H₄yield (a), C-atom selec-

tivity (b), and H-atom selectivity (c) at varying C₂H₆/O₂ratio. Operating

C₂H₆/O₂= 1.6–1.7, which in our experiments is shifted to- conditions: C₂H₆: 24%; total ﬂow rate: 72 Nl/h; T_preheat= 400^◦C.

`

DONSI, PIRONE, AND RUSSO

56

At lower temperature (200–360^◦C), the system reaches

T₁

T₂

T₃

T₄

the stationary condition at very low conversion of ethane

(lowerthan1%)andoxygen(lowerthan4%)andsolidtem-

peratures slightly higher than the preheating one (within

5^◦C). Under those conditions, CO₂was the only product of

ethane oxidation. In the range of preheat temperature

around 380–400^◦C, the ignition of the monolith, charac-

terised by large conversions and temperatures raises, was

observed.

1500

1000

a.

T_adiab

T₂

T₃

T₄

800

600

Figure 4 presents the behaviour of reactor temperature

proﬁle and outlet gas composition during the ignition tran-

sient period, as functions of time after a step change of

ethane concentration in the feed. The behaviour of the

four temperatures measured in the different axial positions

shows that light off occurred in the exit sections of the cata-

lyst; subsequently, the reaction front moved towards the

inlet. The downstream monolith was heated by the cata-

lyst exit section and its temperature (T₄) was always 100^◦C

lower than catalyst T₃temperature. Stationary conditions

were reached in about 2 h. A similar behaviour was ob-

served on the same catalytic system during the transient

of CH₄combustion under lean conditions (26). The time

needed to stabilize temperatures and consequently stabi-

lize conversion and product distribution is probably de-

termined by the large thermal capacity of the solid, as

recently described and evaluated (28). The maximum tem-

perature reached in the inlet sections of the catalytic mono-

lith was 960^◦C; this sharply decreased to 800^◦C in the exit

sections within 2 cm distance, due to either the occurrence

of endothermic dehydrogenation reactions of ethane (20)

or to the defective insulation of the system (28). Indeed,

these values are signiﬁcantly lower than T_adiabreported

in Fig. 4a, which is the value expected after an adiabatic

temperature rise (1250^◦C at steady state). A rough esti-

mate of the adiabaticity degree under the experimental

conditions of Fig. 4 is 65%, i.e., not very different from

the value of 75% reported by Huff et al. for foam mono-

liths (18).

T₁

400

100

b.

O₂

C₂H₆

80

60

40

20

C₂H₄

0

70

c.

C₂H₄

60

30

CO₂

20

10

CO

C₂H₂

CH₄

0

60

d.

HC

50

40

30

20

10

0

H₂O

H₂

The concurrent behaviour of the gas-phase composition

is also shown in Fig. 4: in the ﬁrst minutes of the run, O₂

very quickly reached 100% conversion, while ethane was

mostly converted to CO₂and water. When the solid temper-

ature proﬁle assumed values comparable to those reached

in the stationary regime, the distribution of products slowly

changed, with the selectivities of CO₂and H₂O decreasing

and those of CH₄, H₂, and CO increasing in time. More-

over, also ethane conversion and, consequently, the yield

to ethylene signiﬁcantly increased in the transient period

before steady state.

0

3

6

9

12

15

18 50 100 150 200

time (min)

FIG. 4. Thermal proﬁles at the light off (a). T₁and T₄are placed in

the upstream and downstream shield monolith, respectively (at 0.5 cm

from the catalyst); T₂and T₃are placed in the catalyst monolith at 1 cm

from the inlet and 1 cm from the outlet, respectively; T_adiabis the expected

value of temperature for the outlet gas under adiabatic conditions. O₂and

C₂H₆conversion and C₂H₄yield (b), C-atom selectivity (c), and H-atom

selectivity (d). Operating conditions: C₂H₆/O₂/N₂= 24/16/60; ﬂow rate:

72 Nl/h; T_preheat= 400^◦C.

In the ignited state, O₂was completely consumed

while ethane conversion reached about 84%. The main

C-containing product was C₂H₄with a selectivity of about a concentration about double with respect to the latter in

65% (the overall yield was 55%); also CO₂, CO, and in the steady state.

lower extent CH₄and C₂H₂were produced. Moreover,

Once the reactor was ignited, the preheat could be low-

H₂O and H₂were also formed, with the former exhibiting ered to 240^◦C before extinction occurred. Figure 5 shows

ETHANE ODH OVER PEROVSKITE-BASED MONOLITH

57

Preheat temperature (°C)

250 275 300 325 350 375 400

60

50

40

30

20

10

100

80

O₂

HC

C₂H₆

60

H₂O

C₂H₄

40

20

H₂

a. c.

d.

0

70

0

b.

1000

C₂H₄

T₂

900

800

700

600

500

400

300

60

T₃

CO₂

CO

15

10

5

T₄

T₁

CH₄

C₂H₂

0

250 275 300 325 350 375 400

Preheat temperature (°C)

FIG. 5. O₂and C₂H₆conversion and C₂H₄yield (a), C-atom selectivity (b), H-atom selectivity (c), and solid temperature proﬁles (d) at varying

preheat temperature. T₁, T₂, T₃, and T₄as in Fig. 4. Operating conditions: C₂H₆/O₂/N₂= 24/16/60; ﬂow rate: 72 Nl/h.

that conversions and product distribution were not severely

Even though reactor extinction occurred for a preheat

affected by a 150^◦C decrease in preheat temperature. In- below 240^◦C, Fig. 6 shows that already from T_preheat

=

deed, there is no clear evidence that preheat affected the 255^◦C the reaction front moved towards the exit of the

maximum temperature of the system. It can be supposed reactor. Moreover, notice in Fig. 6 that the temperature

that the maximum temperature was self-adjusted to about T₁, which is the most affected by the decrease in preheat

1000^◦C, because at this temperature heat dispersion be-

comes much more efﬁcient due to radiation. Temperature

1000

T₁in the radiation shield, instead, diminished uniformly

with decreasing preheat temperature.

When the preheat was reduced, a longer distance of the

240°C

reactor worked as a gas preheater, until the threshold tem-

perature of the exothermic reactions was reached. Extinc-

tion occurred when the exothermic reactions took place

over a distance that was too short in the catalytic monolith,

since the entrance sections were too cold and heat produc-

tion was no longer able to balance heat losses.

On reducing preheat, ethane conversion weakly de-

creasedfrom83to80%, whileselectivitytoethyleneslightly

increased. This is possibly due to a positive effect of tem-

perature on ethylene selectivity, even if different factors

can inﬂuence it. As preheat temperature is varied, indeed,

not only do maximum temperature and temperature pro-

ﬁle change but also contact and residence time and ethane

conversion.

800

600

400

400°C

350°C

Decreasing Preheat

Temperature

305°C

255°C

Radiation

shield

Radiation

shield

Catalytic monolith

0

10 20 30 40 50 60 70 80

Reactor Depth (mm)

FIG. 6. Temperature proﬁles in the reactor at varying preheat tem-

perature. Operating conditions as in Fig. 5.

`

DONSI, PIRONE, AND RUSSO

58

temperature, is reduced to about 400^◦C, i.e., very close to heat (500^◦C). At the same feed ratio, even though at lower

the light-off temperature. So, feeding the C₂H₆/O₂mixture conversion, due to lower preheat (400^◦C) and high dilu-

at temperatures lower than this value made the system un- tion (N₂= 60%), we obtained 70% ethylene selectivity.

able to self-sustain the ignited state, as already observed Chromium-based monoliths, although tested under differ-

during the heating cycle.

ent conditions (21), showed performances worse than ours

In contrast, Yokoyama et al. (8) found a strong depen- in terms of yield and selectivity, with scarce stability under

dence of conversion and selectivities on preheat temper- reaction.

ature. Ethane conversion and C₂H₄selectivity were en-

hanced by preheated feed mixture, since preheat raises the

autothermal reaction temperature and provides heat for

thermal dehydrogenation reactions. Results not compara-

ISOTHERMAL MEASUREMENTS

Preliminary blank tests in a 7-cm-long cordierite mono-

ble with ours were reported in the study of ethane ODH lith, carried out under experimental conditions identical to

in an autothermal conventional ﬁxed-bed reactor (16). It those investigated in the presence of the perovskite mono-

was found that reactor temperature increased from 850 liths, allow us to exclude any reaction from proceeding up

to 950^◦C as oven temperature was increased from 300 to to 600^◦C, either in steel tubes or in cordierite shields. Simi-

500^◦C. This leads to increased ethane conversion, from 62 to lar tests were performed by Beretta et al. (15) on uncoated

79%, but not ethylene selectivity, which did not change sig- mullite tubes and conﬁrmed a range of temperatures for

niﬁcantly. However, operating a packed-bed reactor can be the occurrence of gas-phase reactions very close to what

difﬁculty under autothermal conditions, since the high reac- we found. Products of ethane homogeneous oxidation were

tant ﬂow rates needed to achieve quasi-adiabatic conditions mainly ethylene and carbon monoxide, as already put into

lead to unsustainable pressure losses and, consequently, evidence (15, 17).

the lower adiabaticity must be compensated by a larger

preheat.

In order to characterise the properties of perovskite cata-

lysts in the absence of any other chemicophysical effect or

Experimental data (5) reported for an ethane/oxygen homogeneous reaction, a smaller monolith was tested un-

mixture in Pt-based foam monoliths were similar to our re- der almost isothermal conditions. For this purpose, a 0.5%

sults, with conversion and selectivities that did not change ethane mixture, with C₂H₆/O₂= 1.7, was fed into the re-

markedly by lowering the preheat. This can be attributed to actor. As shown in Fig. 7, the solid temperature was kept

the maximum temperature, which, when around 1000^◦C, is below 600^◦C in all runs, and traces of H₂,which indicate a

not affected by further preheat, due to the very effective ra- homogeneous phase chemistry, were never detected.

diative heat transfer mechanisms. Nevertheless, the whole

reactor is slightly hotter and this inﬂuences the thermody- perature, when O₂is not yet completely consumed and

namically favoured products, such as CO, CH₄, and H₂. under conditions that allow homogeneous reactions to be

It is worth noting that C₂H₄is also produced at low tem-

In conclusion, a general statement on the effect of pre- excluded. The amount of C₂H₄produced is very low, be-

heat temperature cannot be expressed. The inﬂuence of this cause both inlet concentration of ethane and conversion

parameter seems to be strongly connected to the degree of and selectivity are low. The sensitivity of the GC appara-

adiabaticity of the laboratory-scale reactor.

tus used for the analysis of gas-phase composition is high

With an ethane/air mixture, perovskite-based catalysts enough to detect parts per million of ethylene with good

seem to perform much better than Pt-based foams, with the conﬁdence.

same preheat temperature. Under similar conditions Huff

The comparison with experimental data reported by

and Schmidt (5) reported less than 55% selectivity to ethy- Beretta (15, 19) over a Pt catalyst under similar controlled

lene and an ethane conversion slightly higher than 80%. conditions shows that the properties of our catalyst are

Only by reducing N₂dilution to 20% and consequently rais- signiﬁcantly different, since it results intrinsically active in

ing ethane concentration from 25 to 50%, probably due to ethane ODH, even if ethylene selectivity is low in this range

a strong temperature increase, could conversion and selec- of temperature. Actually, perovskites are known to be po-

tivity be signiﬁcantly increased to values close to ours.

tential catalysts for the oxidative dehydrogenation of light

Perovskites also seem to exhibit better performance in alkanes, due to the capability to promote the formation of

comparison to active phases other than Pt. In experiments alkyl radicals on their surface, rich in structural oxygen de-

performed over BaMnAl₁₁O₁₉catalyst Beretta and Forzatti fects (29). Over Pt, indeed, only CO₂at low temperature

(22) always obtained ethylene selectivity and ethane con- and CO at higher temperature are produced, while ethy-

version lower than ours, with a preheat temperature of lene is formed above 650^◦C, at the onset of homogeneous

500^◦C. In a rare earth oxide packed bed Mulla et al. (16) reactions.

were able to reach an ethylene yield higher than 50%

This allows us to draw the conclusion that also under

(S_{C H}= 66%) for a C₂H₆/O₂ratio of 2, but had to com- autothermal conditions, characterised by high reactant con-

2

4

pensate for the lower exothermicity of their fuel-rich mix- centrations, good insulation, and consequently high tem-

ture with very low N₂dilution (25% vol) and high pre- peratures, the catalyst should contribute to the formation

ETHANE ODH OVER PEROVSKITE-BASED MONOLITH

59

Oven Temperature (°C )

400 425 450 475 500 525 550

80

60

40

20

100

80

60

40

20

a.

c.

H₂O

O₂

C₂H₆

C₂H₄

HC

100

80

60

40

20

0

d.

b.

25

∆T_adiab

20

15

10

5

CO₂

∆T₃

∆T₄

C₂H₄

∆T₂

∆T₁

0

CO

400 425 450 475 500 525 550

Oven Temperature (°C)

FIG. 7. O₂and C₂H₆conversion and C₂H₄yield (a), C-atom selectivity (b), H-atom selectivity (c), and difference between solid and oven temper-

atures (d) at varying preheat temperature. T₁, T₂, T₃, and T₄as in Fig. 4. Operating conditions: C₂H₆: 0.5% vol; total ﬂow rate: 86 Nl/h; C₂H₆/O2 : 1.7.

of ethylene, even if it seems clear that a large fraction of it ence of Pt. These results were also supported by the kinetic

is produced via gas-phase reaction paths. Hence the weak modelling of gas-phase reactions, which demonstrated that

activity of perovskites could be beneﬁcial to ethylene for- in a purely homogeneous reactor the selectivity to ethylene

mation by a mechanism that can be purely heterogeneous is even higher than in the catalytic tests at the same level of

or hetero-homogeneous, through ethylene formation in the ethane conversion (19).

gas phase, following ethyl radical desorption from the cata-

lytic surface.

As reported in a recent paper (30), we have already ex-

perienced homogeneously controlled reaction systems. The

investigation of ethane ODH in a packed-bed reactor ﬁlled

with different basic oxide catalysts (promoted and unpro-

moted MgO, Sm₂O₃, and La₂O₃) showed that ethylene se-

DISCUSSION

The behaviour of short-contact-time reactors for the ox- lectivity was largely independent of the catalyst composi-

idative dehydrogenation of ethane has been discussed in tion and mainly determined by gas-phase reactions, even at

both experimental (5, 8–16, 21, 22) and theoretical (18–20, a temperature as low as 700^◦C. In a very recent work, Mulla

23) works. Most recent investigations reveal that homoge- et al. (17) clearly conﬁrmed that a high ethylene yield is ob-

neous reaction paths cannot be neglected (18–20), even at tainable both in the presence and in the absence of a packed

very short reaction times. Ethylene production in Pt-based bed of Sr_1.0La_1.0Nd_1.0O_xparticles. Nevertheless, the devel-

foam monoliths seems to occur via a hetero-homogeneous opment of a completely homogeneous process is inhibited

reaction mechanism, in which the catalyst should act as an by the formation of coke in the temperature range of inter-

igniting agent, active for total oxidation reactions, while est, notwithstanding the mild oxidising conditions adopted.

ethylene-forming reactions mainly occur in the gas phase As a consequence, it may be argued that the presence

(9, 15, 18, 19).

of any catalyst could be useful in driving an autothermal

On the other hand, Beretta et al. (15) showed that appro- process but that it reduces the maximum yield attainable

priate preheating of the C₂H₆/O₂mixture in the absence of through heating a C₂H₆/O₂mixture in the absence of the

the catalyst led to even higher C₂H₄yield than in the pres- catalyst.

`

60

DONSI, PIRONE, AND RUSSO

If all this is true, use of very expensive noble-metal-based lower reactant consumption. Moreover, in contrast to what

catalyst should not be necessary. Nevertheless, previous was shown by Beretta et al. (15, 19) for Pt over perovskite-

studies carried out on materials other than noble metals (16, based waschcoated monoliths, it is also possible to hypoth-

21, 22, 29) did not produce any relevant results since the sig- esise that a nonselective ethylene formation is catalysed

niﬁcant limitations encountered, such as the very poor sta- (about 25% selectivity measured at 550^◦C, Fig. 7). Both re-

bility of Cr₂O₃-based foam monoliths (21) or low activity of actions of ethane catalytic oxidation (leading to CO₂and

BaMnAl₁₁O₁₉hexa-aluminates (22), prevented any future ethylene as products) contribute to sustain reactor temper-

application. Instead, the use of honeycomb-type LaMnO₃- ature levels. Nevertheless, the perovskite-based catalyst ex-

based catalyst allowed us to obtain stable and even higher hibitsaquitenegligibleactivitytowardstheweaklyexother-

ethylene yield than those reported on Pt-containing foams mic partial oxidation of ethane to CO and H₂, in contrast

under the same experimental conditions (5, 22). This led to to what is observed over Pt catalysts (9, 21, 22).

two main consequences.

We believe gas-phase reactions play the main role in the

First, testing a reacting system, with a behaviour com- production of ethylene. At this stage, determining the pro-

parable to that of Pt in terms of performance, but where portion between homogeneous and heterogeneous contri-

the most signiﬁcant parameters, such as the active phase butions to the formation of C₂H₄is difﬁcult, but we are con-

and the reactor morphology, have been changed, conﬁrms vincedthattheyieldsattainedinthepresenceofanycatalyst

that in short-contact-time reactors for ethylene production are of the same order of magnitude as those expected in a

the role of the catalyst is not in catalysing the ethylene for- pure homogeneous phase, as proposed or shown by others

mation, but rather in thermally igniting the hydrocarbon/ (15, 17, 19). However, in contrast with the assumptions of

oxygen mixture, via the combustion of a fraction of ethane Bodke et al. (9), we think that the main pattern of ethy-

to CO_xand water (13, 15, 16). In this way, the system can au- lene formation in the gas phase is the oxidative pyrolysis of

tothermally reach the range of temperatures in which gas- ethane, as it is known to proceed with a higher rate than the

phase reactions favourably occur to transform ethane into dehydrogenation into ethylene and hydrogen. More specif-

ethylene without coke formation, probably due to a posi- ically, it seems certain that the ultimate step in ethylene for-

tive effect of the presence of catalyst and/or the very low mation in the gas phase is the decomposition of the ethyl

residence time of hydrocarbons in the high-temperature radical:

zone.

C₂H₅· → C₂H₄+ H·.

[1]

Second, it can be argued that the main reason why noble

metals, and in particular platinum, have been thoroughly

investigated is likely that they are very active catalysts

in the total oxidation of hydrocarbons. Combustion cata-

lysts other than noble metals, such as the signiﬁcantly

cheaper transition metal oxides, suffer poor thermal resis-

tance above about 800^◦C (21) but are conceptually as able

as Pt. As previously demonstrated (25, 26, 28), perovskite-

based catalysts show, together with a good oxidation activ-

ity, an excellent thermal stability and are competitive with

more expensive noble-metal-based catalysts.

Under the presence of oxygen, ethyl radicals are more

quickly formed via an oxidative pyrolysis step,

C₂H₆+ OH· → C₂H₅· + H₂O,

than via thermal dehydrogenation,

[2]

C₂H₆→ C₂H₅· + H·.

[3]

Moreover, in comparison with most of the results re-

ported on Pt-based catalysts (5, 9–12), we found in our ex- In the stationary regime the ethane/oxygen mixture enters

periments not only a higher ethylene yield (comparing the the reactor relatively cold but, as evaluated by Zerkle et al.

results obtained under the same experimental conditions) (20), very quickly reaches the high solid temperature, at

but also a larger CO₂selectivity (always above 10%). This which reaction [2] is much faster than reaction [3].

difference appears relevant, as at the reactor temperature

As a consequence, under the examined reaction condi-

(between 800 and 1000^◦C) the formation of CO as a prod- tions, we think that sketching the reactor as consisting of

uct of ethylene oxidation is largely favoured amongst CO_xtwo consecutive zones, characterised by the presence or ab-

by homogeneous kinetics (18–20). Following the concepts sence of O₂, as proposed by Bodke et al. (9), and accepted

proposed by Bodke et al. (9), Zerkle et al. (20), and Huff with minor modiﬁcations by Zerkle et al. (20), is not a thor-

et al. (18) of a two-zone reactor and “sacriﬁcial ethane,” ough description of the system. Rather than a scheme of in-

it now seems that our catalyst plays the same role of plat- series reactions, we think that a mechanism of parallel steps

inum but with higher efﬁciency, since it sacriﬁces less ethane of catalytic and homogeneous oxidation of ethane is more

than over Pt-based and hexa-aluminate catalysts (9, 21, 22), reliable, where the role of the catalyst, in the case of per-

provided that its catalytic oxidation mainly proceeds to ovskite oxides, could be signiﬁcant not only in igniting gas-

CO₂, with the consequent maximum production of heat and phase reactions but also in forming a fraction of ethylene.

ETHANE ODH OVER PEROVSKITE-BASED MONOLITH

61

CONCLUSIONS

7. Torniainen, P. M., Chu, X., and Schmidt, L. D., J. Catal. 146, 1

(1994).

8. Yokoyama, C., Bharadwaj, S. S., and Schmidt, L. D., Catal. Lett. 38,

181 (1996).

9. Bodke, A. S., Henning, D., Schmidt, L. D., Bharadwaj, S. S., Maj, J. J.,

and Siddall, J., J. Catal. 191, 62 (2000).

10. Huff, M. C., and Schmidt, L. D., AIChe J. 42(12), 3484 (1996).

11. Bodke, A. S., Bharadwaj, S. S., and Schmidt, L. D., J. Catal. 179, 138

(1998).

12. Schmidt, L. D., and Huff, M., Catal. Today 21, 443 (1994).

13. Lødeng, R., Lindva˚g, O. A., Kvisle, S., Reier-Nielsen, H., and Holmen,

A., Appl. Catal. A 187, 25 (1999).

The oxidative dehydrogenation of ethane to ethylene

in a short-contact-time reactor can be successfully carried

out over catalysts other than Pt or Pt–Sn. Use of much

cheaper honeycomb-type cordierite monoliths consisting

of a LaMnO₃active phase supported onto a La-stabilised

γ -Al₂O₃washcoat allowed us to obtain even higher ethy-

lene yield than those reported under the same experimental

conditions over noble-metal-based foams.

These results can be explained in terms of the role of

the catalyst, which is mainly to thermally ignite the hydro-

carbon/oxygen mixture, via the combustion of a fraction of

ethane to CO₂and H₂O. In addition, we also showed that

the perovskite-based catalysts we have proposed, in addi-

tion to their good oxidation activity and excellent thermal

stability, are also intrinsically active in the ethane ODH.

Under the examined reaction conditions, the formation

of ethylene can be considered to occur through two dif-

ferent parallel mechanisms, one catalysed by monolith sur-

faces and the other proceeding through gas-phase radical

reactions.

14. Beretta, A., Ranzi, E., and Forzatti, P., in “Studies in Surface Science

and Catalysis” (A. Corma, F. V. Melo, S. Mendioroz, and J. L. G. Fierro,

Eds.), Vol. 130, p. 1913. Elsevier, Amsterdam, 2000.

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(2001).

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73 (2002).

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191, 46 (2000).

19. Beretta, A., Ranzi, E., and Forzatti, P., Chem. Eng. Sci. 56, 779 (2001).

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J. Catal. 196, 18 (2000).

21. Flick, D. W., and Huff, M. C., Appl. Catal. A 187, 13 (1999).

22. Beretta, A., and Forzatti, P., J. Catal. 200, 45 (2001).

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24. Sadykov, V. A., Pavlova, S. N., Saputina, N. F., Zolotarskii, I. A.,

Pakhomov, N. A., Moroz, E. M., Kuzmin, V. A., and Kalinkin, A. V.,

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Article Doi

Oxidative dehydrogenation of ethane over a perovskite-based monolithic reactor

DOI: 10.1006/jcat.2002.3636

Source and publish data:

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Article abstract of DOI:10.1006/jcat.2002.3636

Full text of DOI:10.1006/jcat.2002.3636

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