Synthesis of ethane and ethylene from methane and carbon dioxide over praseodymium oxide catalysts

~

A PT PA LE IY DSS CA L I A: GENERAL

ELSEVIER

Applied Catalysis A: General 156 (1997) 43-56

Synthesis of ethane and ethylene from methane and carbon dioxide over praseodymium oxide catalysts Kenji Asami*, Ken-ichi Kusakabe, Naobumi Ashi, Yasuo Ohtsuka Research Center for Carbonaceous Resources, Institute for Chemical Reaction Science, Tohoku University, Sendai 980-77, Japan

Received 24 July 1996; received in revised form 29 October 1996; accepted 1 November 1996

Abstract Catalytic effectiveness for the synthesis of ethane and ethylene from methane and carbon dioxide has been examined mainly at 1123 K over 14 lanthanide oxides. Praseodymium (Pr) and terbium oxides show high yield and high selectivity of C2 hydrocarbons among lanthanide oxides. The catalytic performances of Pr oxides are influenced by the preparation method; a commercial Pr oxide prepared from the oxalate shows the highest C2 selectivity of about 50%, and the stable performance for 20 h. Higher partial pressures of C O 2 and CH 4 are favorable for Cz formation over the oxide catalyst. Reaction mechanisms over Pr oxides are discussed in terms of a redox mechanism involving the unstable lattice oxygen. Keywords: Methane; Ca hydrocarbons; Carbon dioxide; Praseodymium oxide

1. Introduction Direct synthesis of ethane and ethylene (C 2 hydrocarbons) from methane is one of the most attractive methods for the efficient use of natural gas as a chemical resource. Some workers have reported that C2 hydrocarbons can be produced directly and catalytically from methane and carbon dioxide through the following reactions [1-3]:

2CH4

+ C O 2 = C 2 H 6 -~- C O ~- H 2 0

2CH4 +

2 C O 2 = C 2 H 4 q-

2CO +

2H20

(1) (2)

* Corresponding author. Present address: Department of Applied Chemistry, Faculty of Engineering, Osaka City University, Osaka 558, Japan. Tel.: +81 6 6053080; fax: +81 6 6902743; e-mail: [email protected]. 0926-860X/97/$17.00 © 1997 Elsevier Science B.V. All fights reserved. PII S0926-860X(96)00408-5

44

K. Asami et al./Applied Catalysis A: General 156 (1997) 43 56

However, no criteria for catalyst selection has been established so far. Therefore, we have examined the catalytic effectiveness of a large variety of unsupported metal oxides including alkaline earth, rare earth, and transition metal oxides for C2 formation from CH4 and CO2 [4-6], and shown that rare earth oxides, in particular, lanthanide (Ln) oxides are more effective [4,6]. The present work seeks to clarify for the first time the activity sequence among 14 Ln oxides in detail, and focuses mainly on examining the catalytic performance of praseodymium (Pr) oxide by changing both the method of catalyst preparation and reaction conditions, since Pr oxide shows high activity and selectivity for C2 formation.

2. Experimental 2.1. Catalyst materials and preparation Commercially available, highly pure (>99.9%) Ln oxides of 14 elements from lanthanum to lutetium, except for promethium, were used as catalysts without any support materials. For praseodymium catalysts three kinds of oxides, abbreviated as Pr(A) to Pr(C), were employed. The Pr(A) and Pr(B) oxides were commercially available; the latter was prepared by calcining Pr oxalate in air at 1373 K, and the former was prepared by reducing it in H2 at 973 K. The Pr(C) oxide was obtained by decomposition of Pr(NO3)3.6H20 at 1123 K in He. "Pr" represents Pr(A) catalyst, unless otherwise stated. All the Ln oxides in powdery forms were first pelletized and then crushed to 0.4-0.7 mm [4].

2.2. Catalytic runs and product analysis The reaction of CH4 and CO2 was performed under atmospheric pressure with a quartz-made tubular reactor with 11 mm i.d. After a given granular Ln oxide was calcined in the reactor in an air flow (100 ml/min) at 1123 K for 1.5 h, the air was replaced with highly pure N 2 (>99.999%) at 1123 K. During this replacement a significant amount of 02 evolved from Pr and terbium (Tb) oxides. After the evolution of O2 stopped, an atmospheric mixture of CH 4 and CO2 was introduced through the catalyst. The reaction temperature was controlled by a thermocouple inserted into the catalyst bed. The effluent from the reactor, containing reaction products such as C2H6, C2H4, CO, and H2, was sampled at a predetermined interval and analyzed with two on-line gas chromatographs attached with flame ionization and thermal conductivity detectors; hydrocarbons and inorganic gases were separated with Porapak Q and activated carbon columns, respectively. The details of the procedures and product analysis have been described elsewhere [4].

IC Asami et al./Applied Catalysis A: General 156 (1997) 43-56

45

The catalytic runs were carried out under the following conditions unless otherwise stated: temperature, 1123 K; total pressure, 101 kPa; molar ratio of CH4 to CO2, 1.0; total flow rate, 100 ml(STP)/min; weight of catalyst, 2.0 g.

2.3. Data processing Data processing has been described in detail elsewhere [4] and is thus simply explained below. Conversion of CH 4 and CO2, product yield, and product selectivity were calculated based on some following assumptions. The C or H in CH4 is converted either to C2H6, C2H4, and CO or to H 2 and H20, respectively, and the C in CO2 to CO alone. Eqs. (1)-(4) can be regarded as the overall reactions by taking into account most of the possible side reactions, for example, the reverse water-gas shift reaction, steam reforming of CH4, and dehydrogenation of ethane [4]. CH4 + 3CO2 = 4CO + 2H20

(3)

CH4 + CO2 = 2CO + 2H2

(4)

The CO observed arises from two sources of CH4 and CO2, and the CO from CH 4 can be calculated by the stoichiometry of the above four equations [4]. Yields of C2H6, C2H4, and CO formed from CH4 are expressed in mol% of the C in CH4, and the sum of these yields equals CH 4 conversion. Product selectivity can be obtained by dividing product yield by CH4 conversion. Initial conversion, yield, and selectivity at 10 min after the start of the reaction will be discussed unless otherwise stated. The reproducibility of the results was within ±5%.

2.4. X-ray diffraction analysis Crystalline phases of Ln oxide catalysts were determined by the powder X-ray diffraction (XRD) analysis. The XRD measurements were made by using Ni-filtered Cu-K~ radiation.

3. Results

3.1. Catalyst sequence among Ln oxides Initial conversions of CH 4 and C O 2 a r e shown in Fig. l, where the metal element denotes the corresponding oxide. The sequence of each conversion among Ln oxides examined was quite similar; Ce>Eu>Pr~Tb~Tm for CH4 conversion and Ce>Eu>Tb~Pr~-,Tm for CO2 conversion. The Ce oxide gave the highest CH4 and CO2 conversions of 14% and 30%, respectively, and showed a comparable activity to those observed over Cr and In oxides

46

K. Asami et al./Applied Catalysis A: General 156 (1997) 43-56

30 0

tO

2O

0 tO 0

d o

0 15

O

10 c O

.3 tO O

£ o La Ce Pr Nd SmEuGd Tb Dy H0 Er TmYb Lu Lanthanide element

Fig. 1. Conversions of carbon dioxide and methane over lanthanide oxides at 1123 K. The experimental error was within ~4.8%.

[4]. On the other hand, C H 4 conversions over other Ln oxides were less than 5%. The conversion ratio of CO2 to C H 4 w a s approximately 2 with all the Ln oxides, which indicates the overall reaction of CO2 and C H 4 with a molar ratio of about 2. Fig. 2 shows initial yield and selectivity of C2 hydrocarbons. C2 hydrocarbons were formed over all the oxides, but the yield was dependent on the kind of the oxide. Pr and Tb oxides showed higher C2 yields (1-1.5%). C2H6 yield was higher than C 2 H 4 yield over these catalysts, and the proportion of C2H6 in C2 hydrocarbons was 65-70%. The selective formation of C 2 H 6 w a s also observed with other oxides except for Ce and Eu oxides. Fig. 2 also shows that Pr, Nd, Tb, and Dy oxides give higher C2 selectivity (50-60%). On the other hand, the lowest selectivity was observed over Ce oxide with the highest CH4 conversion. These observations point out that Pr and Tb oxides have higher catalytic performance for C2 formation. C H 4 conversion and C2 selectivity over Pr and Tb oxides were unchanged during 4 h reaction. The change in C2 yield with time on stream over Pr catalysts will be described later in more detail.

K. Asami et al./Applied Catalysis A: General 156 (1997) 43-56

47

60

O

E I

40

>.,

,> "5

q

20

69

v

C2H4 o

1.0

E

C2H6

"o

-~ 0.5 >-

0 ~

La Ce Pr Nd SmEu GdTb Dy Ho Er TmYb Lu Lanthanide element Fig. 2. Yield and selectivity of within :c3.1%.

C2

hydrocarbons over lanthanide oxides at 1123 K. The experimental error was

3.2. Effect of the method of preparation on the performance of Pr oxide catalysts Figs. 3 and 4 show the changes in C2 and CO yields with time on stream over three Pr oxides prepared by different methods, respectively. The Pr(C) catalyst derived from Pr nitrate exhibited the highest C2 yield which increased with time. CO yields over Pr(B) and Pr(C) were almost the same, and the lowest yield was observed over Pr(A). CO yield was almost unchanged with time in any cases. Thus, the performance of Pr oxides depended on the method of catalyst preparation, and the Pr(A) and Pr(C) oxides were the most selective and active catalyst, respectively. The fate of the carbon and hydrogen in CH4 reacted is shown in Fig. 5, where the data at 4 h are presented, and the upper and lower bars denote the product selectivity on the basis of carbon and hydrogen, respectively, the amount of H20 being determined by difference. The sequence of C2 selectivity was Pr(A)>Pr(C)>Pr(B), the highest value on the basis of carbon being about 50% over Pr(A) catalyst. The preferential formation of C2H 6 was observed irrespectively of the kind of Pr catalyst, as shown in Fig. 2. More than 60% of the hydrogen

K. Asami et al./Applied Catalysis A: General 156 (1997) 43 56

48

2

~ K/'~

0

E[ 0

Pr(C)

1

L

~

/

-

"o (b >,, o,i

-o------o Pr(A)

~

Pr(B)

0

0

1

2

3

4

5

Time on stream (h) Fig. 3. Change in C2 yield with time on stream over Pr oxide catalysts at 1123 K.

4

o"9. 3 O

EI O

•

,/,

I

J

v

2

Pr(B)

o t

0 0

Pr(Ao) 1

I

0

1

I

2 3 4 Time on stream (h)

5

Fig. 4. Change in CO yield with time on stream over Pr oxide catalysts at 1123 K. The experimental error was within -+-5.0%.

K. Asami et al./Applied Catalysis A: General 156 (1997) 43-56

49

[]CzH6, mC2H4,I~CO, EIH2, [] H20 i

Pr(A)

Pr(B)

C

J

H C

Pr(C)

0

20 40 60 80 Product selectivity (%)

1O0

Fig. 5. Fate of the C (upper histogram) and H (lower histogram) in methane reacted at 1123 K for Pr oxides.

in CH4 was converted to water in every case, and H2 was the minor product over each catalyst. These results show that the major overall reactions proceed through Eqs. (1) and (3). Fig. 6 shows the profiles of C2 and CO yields with a prolonged time on stream. Both yields with the most selective Pr(A) catalyst did not change significantly for about 20 h, except for the first 1 h, showing the stable catalyst performance. C2 yield with the most active Pr(C) catalyst showed a maximum at 4 h, and it decreased gradually with time. On the other hand, the profile of CO yield with this catalyst had the reverse tendency. Thus, C2 selectivity decreased from 42% at 4 h to 24% at 48 h.

3.3. Effect of reaction conditions on the performance of Pr catalyst Fig. 7 illustrates the temperature dependence of C 2 formation over Pr(A) catalyst. As the temperature was raised, C2 yield increased and reached 1.9% at 1173 K, whereas C2 selectivity had a maximum at around 1100 K and then decreased. The proportion of CzH4 in C2 hydrocarbons increased from 7% at 1023 K to 56% at 1173 K. CH4 conversion also increased from 0.3% to 5.7%. The influence of contact time (W/F) on the formation of C2 hydrocarbons and CO at 1123 K over Pr(A) catalyst is shown in Fig. 8, where catalyst weight (W) is constant but total flow rate (F) of the reactant is changed. Product yield is illustrated in Fig. 8(A). C2H4 and CO yields increased with increasing W/F, whereas C2H6 decreased gradually with W/F after about 10 g h/mol. The highest

K. Asami et al./Applied Catalysis A: General 156 (1997) 43-56

50

4j

co

0 A C2 0 /~

0

E I o

-o ._~ >-

L

0

J

I

I

I

10

20

3O

40

50

Time on stream (h) Fig. 6. Change in C2 and CO yields with a prolonged time on stream over Pr oxide catalysts at 1123 K.

!100 <

O

E I

0

E I (O v

1

.............-o--

50

._> "5 03 I1) o'1

(O

t'N

O

0 1000

~ 11 O0

1200

0

Temperature (K) Fig. 7. Temperature dependence of

C2

formation over Pr(A) catalyst.

K. Asami et al./Applied Catalysis A." General 156 (1997) 43-56

100

~ E

51

(B)

8o CO

~

60

.~

40i

{3

'~ co

20

(A)

co O

E

2

I

0.9, "O ~

0

H

4

-i1.

I

I

I

10

20

30

W/F

40

(g. h/tool)

Fig. 8. Effectof W/F on the performanceof Pr(A) catalyst.

obtained at about 13 g h/mol. As is seen in Fig. 8(B), as W/Fincrease, C2 selectivity decreased but selectivity ratio of C2H4 to C2H 6 increased. When the curves of selectivity-W/F were extrapolated to W/F of zero, that is, at contact time of zero, selectivity of C2H6, C2H4, and CO seemed to approach 60%, 0% and 40%, respectively. These observations show that parallel and successive reactions of CH4 and CO2 proceed over Pr catalyst; CH4 reacts with CO2 primarily to form in parallel CO and C2H6, which is converted to C2H 4. C2 hydrocarbons formed are further oxidized successively to CO. It is likely that C2H 6 formation proceeds through coupling reactions of methyl radicals, as observed in the oxidative coupling of CH4 with O2. Figs. 9 and 10 show the dependence of partial pressure of CO2 and CH 4 (denoted as P(CO2) and P(CH4)) on the performance of Pr(A) catalyst at 1123 K, respectively. Space time yields of C2 hydrocarbons and CO increased with increasing P(CH4) or P(CO2). Apparent reaction orders for C2 formation with respect to P(CO2) and P(CH4) were 0.4 and 0.8, respectively, whereas those for CO formation were 0.1 and 0.5, respectively. In other words, C2 formation showed a larger pressure dependence with respect to both P(CO2) and P(CH4). Thus, larger C 2 yield was

K. Asami et al./Applied Catalysis A: General 156 (1997) 43-56

52

1.0

1 O0

cO') O

O

E E

E I

I

O

O v 13 09 >,,

v

,C

0.5

o...z~.. . . . . . ~ . . . . . . -z5

50

>"

° m

.>

>

o (3) 09 03 04

09

E

.B

09 ,{3

O

03

0

I

I

I

I

i

10

20

30

40

5O

P(CO2) Fig. 9. Effect of partial pressure of P(CH4), 50 kPa; balance, At.

CO 2 on

0

(kPa)

the formation of C2 hydrocarbons and CO over Pr(A) catalyst.

1.0

1100

tO

O

E E

E I

I

(.9

O

v

-o 0.5

50

09 >, (2)

>"

.i..a ° ~

._> ae.a

o (2) 09 Go 04

E

° ~

(1) o

O

o..

09 I

0

10

I

I

I

20 30 40 P(CH4) (kPa)

Fig. 10. Effect of partial pressure of CH4 on the formation of P(CO2), 50 kPa; balance, At.

C2

I

50

hydrocarbons and CO over Pr(A) catalyst.

K. Asami et al./Applied Catalysis A: General 156 (1997) 43-56

53

Table 1 Crystalline species identified for Pr oxide catalysts by X-ray diffraction analysis. Catalysta

Pr(A)

Pr(B)

Pr(C)

Crystalline speciesb

F B U F B U F B U

Pr203 (1.5) d

PrO1.5+xc (1.5-1.7)

S S S

VW VW

W

S S

W

S S

Pr407 (1.75)

Pr6Oll (1.83)

PrO 2 (2.0)

W

VW

S

S

~F, fresh; B, just before introduction of CH4 and CO2; U, used. bDiffraction intensity designated as: S, strong; W, weak; VW, very weak. ° 0
P(CO2) and P(CH4) lead to higher C 2 selectivity. These results suggest that higher yield and selectivity of C2 hydrocarbons may be attained under the pressurized conditions.

3.4. Crystalline phases of Pr oxide catalysts Table 1 shows the results of XRD measurements for Pr catalysts. "Fresh (F)" means as received or after decomposition of Pr nitrate. With the catalyst "just before the introduction of CH4 and CO2 (B)", the fresh oxide was first calcined in air, then exposed to highly pure N2 until any 02 evolved from the calcined oxide cannot be detected, and finally quenched to room temperature. "Used (U)" means after 4 h reaction of CH4 and CO2 followed by quenching in pure N2. As shown in Table 1, the fresh Pr(A) catalyst existed in the crystalline form of Pr203, which was also the predominant species just before the introduction of CH4 and CO2 and in the used catalyst. In the two samples a new phase with very weak XRD intensities was observed and identified as PrO1.5+x (0
54

K. Asami et al./Applied Catalysis A: General 156 (1997) 43-56

TabLe 2 Phase transformations of Pr oxide catalysts

Catalyst

Atmosphere and temperature Air Air N2 N2 CH4/C02 ~295 K %~" 1123 K -i~- 1123 K ~" ~295 K ~" 1123 K ~ [

N2 1123 K

Pr (A)

Pr203

Pr203 (PRO1.5+x)

Pr203 (PRO1.5÷x)

Pr (B)

PrO2 (Pr6011) (Pr407)

PRO1.5+x

PRO1.5+x (Pr203)

Pr (C)

Pr6011

PRO1.5+x

PRO1.5÷x (Pr203)

comprised the same phases (Pr203 and PrOj.5+x), though the ratio of the two species in XRD intensity was different among the three catalysts (Table 1).

4. Discussion Among 14 Ln elements examined, Pr and Tb showed higher activity for C2 formation based on a fixed catalyst weight, and also gave higher C2 selectivity (Fig. 2). On the contrary, these catalysts have been reported to be less selective for C2 formation in the oxidative coupling of CH 4 with 02 [9-11] because of easy combustion of methyl radicals over these oxides [12]. It is suggested that the present mechanism is different from that in the oxidative coupling. As is well known, Pr and Tb have multiple oxidation states [7,8], and their lattice oxygen atoms are labile [ 13,14]. Thus, it appears likely that the unstable lattice oxygen in Pr and Tb oxides play a crucial role in Ca formation by the reaction of CH 4 and CO2. Upon heating such the lattice oxygen releases as O2, which can be detected quantitatively by the temperature programmed desorption (TPD) method [13,14]. The TPD experiments of Pr(C) catalyst have revealed that the x in PrO1.5+x can be estimated to nearly zero (0.04), in other words, the catalyst just before the introduction of CH 4 and CO2 is almost in the lowest oxidation state [15]. When CH4 alone was introduced in this state, no appreciable amount of C2 hydrocarbons was observed [15]. As shown in Fig. 3, however, the co-feeding of CH4 and CO2

K. Asami et al./Applied Catalysis A: General 156 (1997) 43-56

55

lead to C 2 formation. These observations indicate that C O 2 first activates the Pr catalyst, which then promotes C2 formation. It is likely that the catalyst is first oxidized with CO2 to form the unstable nonstoichiometric oxide in a higher oxidation state, denoted as PrO1.5+y (y>x). The degree of the oxidation would be very small and/or limited to the surface layer, since no significant weight increase was detectable when the equimolar mixture of CH4 and CO2 was passed over the catalyst after a prolonged exposure to pure N2 [6]. As another initiation step, the Pr catalyst may be transformed into the basic carbonate by the reaction with CO2 [ 11,16]. The presence of the carbonate, if any, would make minor contribution to C2 formation, however, since La203 with the largest propensity to basic carbonates among Ln oxides [11] was less active and less selective for C2 formation (Fig. 2). The resulting PrO1.5+y may react directly with C H 4 to yield methyl radicals, which subsequently undergoes coupling reactions to form C2 H 6. Part of the radicals are also converted to CO, as suggested in Fig. 8. Complete combustion of methyl radicals to CO2 proceeds extensively over the higher oxide of Pr6Oll with the O/Pr ratio of 1.83 [12,17]. High C2 selectivity observed over the Pr catalyst (Fig. 2) thus shows that the y in PrO1.5+y may be much smaller than 0.3 and/or the complete combustion can be suppressed in the presence of CO2. In the formation process of methyl radicals PrOt.5+y is regenerated to PrOLs+x and/or reduced to Pr203 (Table 2). Thus, C2 formation may proceed through a redox mechanism involving the unstable lattice oxygen. It is not clear at present why the performance of the three Pr catalysts are different (Figs. 3-5). The Pr(B) and Pr(C) catalysts showed the lowest and highest C2 yield, respectively (Fig. 3), but their crystalline phases just before introduction of C H 4 and CO2 as well as after 4 h-reaction were the same (Table ltable 2). It is therefore evident that the bulk phase in the Pr catalyst cannot correlate with the activity for C2 formation. The X-ray photo electron spectroscopy measurements failed to determine any significant differences in surface compositions of these catalysts. The different method of catalyst preparation before air calcination in the reactor may change the surface morphology, which may in turn affect the performance in the reaction of C H 4 and CO2.

5. Conclusions

The formation of C 2 hydrocarbons from C H 4 and C O 2 o v e r lanthanide oxide catalysts has been examined, and the following conclusions are summarized. 1. All the oxides promote C2 formation at 1123 K, and terbium and praseodymium oxides show high C2 yield and high Cz selectivity. 2. The catalytic performance of Pr oxides depends strongly on the preparation method; Pr oxide prepared by calcining the nitrate in He gives the highest C2

56

K. Asami et al./Applied Catalysis A: General 156 (1997) 43 56

yield, whereas the commercial Pr oxide after air calcination of the oxalate followed by H2 reduction exhibits the highest C2 selectivity. 3. A short contact time gives selective formation of C2H6, which is subsequently converted to C2H 4 as the time increases. 4. Both C2 yield and C2 selectivity over Pr catalyst at 1123 K increase with increasing partial pressure of CH4 or CO2, whereas C2 selectivity decreases slightly beyond 1123 K. 5. Unstable lattice oxygen atoms in Pr oxides are suggested to participate in C2 formation through a redox mechanism.

Acknowledgements Part of this work was carried out as a research project of the Japan Petroleum Institute commissioned by the Petroleum Energy Center with the subsidy of the Ministry of International Trade and Industry.

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