BaCO3 Catalyst

A. Holmen et al. (Editors),Natural Gas Conoerswn 0 1991Elsevier Science PublisheraB.V., Amsterdam

THJ3 PATHWAY OF OXIDATIVE COUPLING OF ME?HANE OVER A

65

b203/%aIS

CATALYST

Xuejia DING, Zhenqiang W, Xiaolai WANG and Shikong SF!W Lanzhou Institute of chemical Physics, Chinese Academy of Sciences, b Z h O U 730000, PR ChiM m T R A c r

The reaction network for the oxidative coupling of methane over 8 w t % catalyst has been determined by varying partial pressures and residence time in temperature range from 720 to 8OOOC using a gradientless reactor. Ethane and most of Cox(mainly C o z ) are the primary prcducts of methane oxidation; ethylene is consecutively fonned from ethane reaction; some of COY are also formed by the secon&ary oxidation of Cz. In combination with the pulse-reaction and transient response experiments, it could be derived that the reversible adsorbed oxysen on the Lazo3/%CO3 catalyst is responsible for activation of methane. Laz03/Baco3

INTRDWCTION

Methane direct oxidative conversion into useful chemicals has attracted much attention of mny researchers. Among many trials in conversion of methane, the direct synthesis of ethylene via oxidative coupling of methane is the most successful and promising way for the time being. Most of the catalysts that have been studied are reducible metal oxides, oxides of rare earth metals, alkali metal oxides and alkaline earth metal compunds[lJ.We have found that Laz03/BaCo3 catalysts exhibit high activity and selectivity for converting methane into Cz hydrocarbons with excellent stability[2,3]. A 8 wt% hzo3/&3al3 catalyst, which is used in the present study, has been found to have no decline in catalytic activity and selectivity in a run exceeding 500 hours on stream in our hbratory[41. This paper reports the results of detailed kinetic experiments carried out with a sample of 8 w t % Laz03/Baco3 using a gradientless reaction system as well as pulse-reaction and transient response technique. EXPERIMENTAL 8 w t % h 2 0 3 / k C o 3 catalyst was prepared by coprecipitation f r m a solution of La(NO3)3 and Ba(NO3)z using (NHi)zCo3 as precipitating agent. The dried catalyst was calcined in air at 8OO0C for 12h and then crushed ard sieved to a grain size of 0.35-0.79mn. BET surface area of calcined catalyst is 3.7mz/g. The kinetic experiments were carried out in a gradientless reactor with respect to gas phase concentration this being achieved by external recirculation of reactive gases using a metal bellows pump. A recirculation

66 ratio more than 30 is used. Water vapor is directly condensed out of the reactor-effluent using an ice trap. The pulse reactions were performed by introducing a gas pulse using six-way sampling valve with 0.2ml loop. Helium with flow rate of 30lnl/min was used as a carried gas to carry the pulses gas via reactor to GC. The transient response experiments were performed using a 4-way valve to switch gas line and a 16-loop storage valve after reactor for sampling. Samples of the reaction products were automatically collected at 5-20 sec. intervals with the 16-loop valve and later analyzed by GC. A same type of quartz reactor(5nm o.d.1 with 25Omg catalyst, which was especially designed to minimize dead volume, was used in the all three reaction systems. The products were analyzed by on-line GC with TCD and 2m porapak Q column and 2m 5A molecular sieve column. The gases were of the following purities: methane(99.99%), ethylene(99.951, nitrogen(99.99%), oxygen(99.8%), helium(99.995%). RESULTS AND DISCUSSIONS Figure 1 shows the dependence of product selectivity on the residence time over h203/bm3 catalyst obtained in a single-Wss fixed bed quartz reactor using methane/air=l as cofeed.

1

0.00 0.02 0.04 0.06 Residence Tine. sec. Fl8.l Methane conversion and selec. of products as a function of residence time. (Tr-760°C, CHr:air-1, P C H r conv.. CzHc S. O C z H r S.,

0

cox s. )

0

-1 . 5

L

0.6 0.9 1.2 Residence Tine, s e c .

0.3

Flp.2 Methane c o n v e r s i o n a n d yield as a f u n c t i o n o f residence t i m e .

(Tr-76O0C. C H I : A ~ ~ - I.CHI , COIIY. QCiHr Y , . aC,Hb Y.. bCOX Y.)

67

02

Flg.3 E t h y l e n e c o n v e r s l o n and y i e l d s a s a f u n c t i o n of r e s i d e n c e tlme. (Tr-760°C,

xco,

Y..

Cplf*:Air-l. oC,HI Y.)

oco

conv.

x

Fig.4 'The e f f e c t o f 02 c o n c e n t . I n

mettsane-oxygen on t h e r e s u l t s o f pulse r e a c t i o n . (Tr-760'C. *CHr conv.,AC?Hr Y.,OC,H' Y..ACOx Y )

The selectivity of ethylene increases considerably with increasing residence time, while the selectivity of COX increases slightly with increasing residence time. The only product displaying increasing selectivity with decreasing residence time is ethane. By the extrapolation of the curves in Fig.1 to zero residence time, the selectivities approach zero f o r ethylene, 68% for ethane and 32% for COX respectively. From these results it could be derived that ethane is primary product of methane oxidation. ethylene is consecutively formed from ethane reaction. Most of COX are formed by direct oxidation of methane, while some of COX may be formed by complete oxidation of C2 products. The resulting reaction scheme being only a rough approximation is shown as below:

The typical dependence of residence time on the yields and conversions is shown in Fig.2 and 3 for reaction of methane-oxygen, and ethylene-oxygen cofeed respectively. First of all, the kinetics of ethylene oxidation to COx(step 5 ) was determined by fitting experimental data from reactions of ethylene-oxygen as cofeed. In combination with the kinetics of ethylene reaction, the overall kinetics of methane oxidation was determined by fitting the experimental data, which included varying partial pressures and residence time at 720, 740, 760, 780 and 8OOOC using linear regression analysis to power law rate equations according to the reaction scheme beneath:

68 ri= 7.81 exp(-42,Okcal/RT) Pm1a5 PoOa5 r2= r3=

r4=

(nmol of ethane/g.h.) (mn01 of CDx/g.h.) 1.48~104 exp(-32.0kcal/RT)Pa Pooa5 (mnol of ethylene/g.h.) 32.3 exp (-23.Okcal/RT)Pa (mn~l of COx/g.h.) 1.05~10-~ exp(-20.Okcal/RT) PD Po

r5= 19.5 exp (-21.5kcal/RT) Pe

(-1

of COx/g.h.)

where r i , 1-2, r 3 , r4 and 1-5 represent respectively the formation rates of each step in correspondence with above reaction scheme, and PD, Pa, Pe and Po are the partial pressure of methane, ethane, ethylene and oxygen respectively(unit in Pa). The rates of each step in Table 1 were obtained by substitution of the practical partial pressures and reaction temperatures to the rate equations as mentioned above. The relative deviations for the rates of each step between the calculated values and the experimental values are less then 20%. The data in Table 1 show that COX came mainly (larger than 90%) from methane rather from the further oxidation of ethane and ethylene. Ethane and ethylene are favored at low oxygen pressure and high reaction temperature. COX are favored at high oxygen pressure and low reaction temperature. The apparent activation energy 42 kcal/mol for formation of ethane observed by this work is quite coincident with twice the apparent activation energy(24 kcal/mol) for forming gas-phase methyl radicals over a Li/MgO catalyst observed using MIESR technique15I . The reason probably is due to ethane forming via the combination of methyl radicals. Table 1 The rate of each step calculated for the reaction scheme according to the rate equations as mentioned above. Formation rate Partial pressure Reaction (mmol of product/g.hr.) ( xlO3Pa 1 Temp. ri rz 13 r4 OC Pm Po Pa Pe 6.72 0.44 0.14 0.04 38.8 4.15 0.51 0.10 2.19 720 0.10 7.06 0.79 0.25 3.04 740 38.0 3.65 0.71 0.20 0.11 9.41 1.28 0.36 4.84 38.6 3.95 0.81 0.20 760 10.60 1.89 0.50 0.20 6.81 780 38.1 3.75 0.91 0.30 2.61 0.60 0.33 10.29 13.70 38.1 4.05 0.91 0.41 800 0.06 1.3.25 0.73 0.13 3.54 760 23.8 9.02 0.30 0.10 0.11 12.44 1.10 0.27 5.62 39.0 5.18 0.61 0.20 760 12.57 1.15 0.31 0.11 7.10 760 49.0 4.15 0.71 0.20 0.17 13 30 1.63 0.49 59.1 3.65 1.11 0.30 8.80 760 The kinetics of ethane dehydrogenation to ethylene with 32 kcal/mol apparent and 0.5 order for activation energy is first order for ethane pressure oxygen. It seem that the ethylene is formed by both homogeneous dehydrogenation and catalytic oxydehydrogenation of ethane. In order to further ascertain the role of the different oxygen species,

69

pulsing reaction and transient response experiments were carried out. The yields and conversions as a function of oxygen concentration in gdse are shown in Fig.4, 5 and 6 for methane-oxygen pulses, ethane-oxygen pulses and ethyleneoxygen pulses respectively.

7 12

k*

.a

u

l5

I

51Lz&2zd 51Lz&&i2 20

0 Q X Fig.5 T h e e f f e c t o f O 1 c o n c e n t c a . in e t h a n e - o x y g e n m l x t u r e o n t h e i s v u l r s of pulse reactlon. ( e C ~ i l r c o n v . O C ~ H rY . , O C H r Y . . A C O , r . TrP7600C.)

10

02 x

Fig.6 The e f f e c t o f 02 c o n c e n t r a t i o n i n e t h y l e n e - o x y g e n mixture on t h e

r e s u l t s o f p u l s e r e a c t i o n . (Tr-760°C C z H s c o n v , , 0 C H q Y., A CzHc Y., ACOx Y.)

There is almost no Cz and COX being obtained either by pulsing pure methane or pulsing alternately methane and air. It means that neither lattice oxygen nor irreversible absorbed oxygen participates in the oxidative reaction. However methane-oxygen pulsing reactions show that the ethane and ethylene yields increase sensitively with increasing oxygen concentration in methane-& pulses.Thus, it seems likely that the reversible absorbed oxygen species are responsible for methane activation. The data from pulsing ethane-02 in fig.5 show that a considerable etbylene is obtained by pllsing pure ethane, and ethylene yields are weakly dependent of oxygen concentration in ethane-az pulses. These features are agreement with the mechanism of ethylene formtion via both homogeneous dehydrogenation and catalytic oxydehydmgenation of ethanen. In addition, considerable amount of methane and hydrogen(n0 showing in Fig.5) is formed by pulsing ethane-oxygen. It could be assumed that the recanbination of methyl radical formed by horoolytic dissociation of ethane with hydrogen radical is one possibility of methane forming way. All pulsing reactions at low oxygen concentration show that only very -11 amount of COX was observed. To understand that, a series of consecutive pulsing reactions with the composition of oz/CH~1=0.08at 7600C were carried out. In this case the interval of 3 minutes between each consecutive pulsing was used, but in normal pulsing reaction the interval of 10 minutes between each &sing reaction was used. The data in table 2 obtained

70 from

::::::H C.C.

II\ r,me(mln)

Fig.7

02-CH4 and CH4-02 responses a t 750'C over quartz ( a ) and 8wt5 LaPO3/BaCO3 catalyst (b) oCH4, o 02, A C Z ~ 6 , a COP. F.120 ml/min.

consecutive

pulsing

reactions show that at first pulsing there is no COX to be formed, at second and third pulsing there are some O r to be formed. These results seem to suggest that COz formed at the first pulsing is easy transformed into Laz(003)3 with La203 on the surface of the catalyst. The IR experiments carried out in our laboratory give evidence for that. The equilibrium between the carbonation and the decarbonation is quickly established under steady state conditions. However, decarbonation of formed Laa (COO1 3 could be slowly

occurred by sweeping He gas. It is conceivable that the decarbonation of formed Laz(O3)3 has extremely occurred after 10 minutes, while decarbonation occurs just W t l y after 3 minutes. Table 2 The results of consecutive pulse reaction of methane-oxygen mixture. Pulse numbers

1

2

3

Yield of 0 3 2 X

0

0.41

0.46

Fig. 7 shows the results of transient response for Oz-methane-02 obtained on the h 2 0 3 / % ~ 3 arxi crushed quartz respectively at 760°C. Despite a exception that a ethane peak appears following sequentially by switching 02-methane, the transient response experiments indicate that the transient curves over Laz03/Baco3 are quite close to that over crushed quartz. It reveals that the kinetic equilibrium of adsorption-desorption between the gas phase oxygen and the adsorbed oxygen is very quick. According to a rough estimation, the amount of adsorbed oxygen is probably less than one monolayer. Copulating with transient curves obtained over crushed quartz, the ethane peak in Fig.7 presents a evidence for methane reaction with reversible adsorbed oxygen. The XPS study of h203/&@ catalyst[6] and EPR study of hzo3[7] have presented

71

evidence for the existences of a superoxide species 0 2 - . However, it still requires further work to clarify what kind of oxygen species exists on the surface of h 2 0 3 / b a I 3 catalyst. CONCLUSION The results obtained in this study have defined the reaction network for oxidative coupling of methane over hzoJ/BaCOS catalyst. Methane direct reacts to form ethane and most of COX as primary product. Ethylene is consecutively formed from ethane reaction. Some of OX is also formed from oxidation of ethane and ethylene. Ethane and ethylene are favored at low oxygen pressure and high reaction temperature. COX are favored at high oxygen pressure and low reaction temperature. The reversible adsorbed oxygen is responsible for activation of methane, while the lattice oxygen of Laz0~/BaCO3 can not participate in the oxidative reaction. REFERENCES [l] G.J. Hutchings, M.S. Scurrell and J.R. Woodhouse, Chem. SOC. Rev., 18, 251(1989). Yu, S. Shen, S . Li and H. Wang , J. Molecular catalysis(China), [2] 3, 181(1989). 131 2. Yu, S. Shen, S. Li and H. Wang, Reprints of 3B Symposium on Methane Activation, Conversion and Utilization, PACIFICHEM'89, p.126, Dec. 1989, Honolulu. [ 4 1 X. Ding, 2 . Yu, X. Wang, S . Liu, S. Shen and S. Li, J. Molecular Catalysis(China), 4, 252(1990). [5] K.D. Campbell and J.H. Lunsford, J. P~Ys. Chem. 2, 579211988). [6] 2 . Jing, Z.Yu, B. Zhang, S. Shen, S. Li and H. Wang, Symposium on " The Production of Ethylene and other Olefins by Nonconventional Mans" Boston, 1990. 171 C.-H Lin, K.D. hpbell, J.-X. Wang and J.H. Lunsford. J.Phys. Chem., 90, 534(1986).