SiO2 catalysts

3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All rights reserved.

T h e Effect of t h e P b O L o a d i n g in Methane over PbO/SiO2 Catalysts.

the

737

Oxidative

Coupling

of

H. J. Lugo, N. Teran, L. Villasmil, G. Castillo and D. M. Finol Centro de Superficies y Cat~lisis, Facultad de Ingenieria, Universidad del Zulia, Apartado 15251, Maracaibo 4003A, Venezuela

The effects of the PbO loading and the C H 4 / O 2 ratio on the oxidative coupling of methane (OCM) over PbO/SiO2 catalysts were studied. Special emphasis was made in the interpretation of the product distribution in the gas phase and its relationship with the nature of the catalyst surface. At CH4/O2 > 2 ratio, the catalytic behavior of the 2 and 6 % PbO/SiO2 catalysts was very similar. The activity for the deep oxidation of CH4 to CO2 and the activity for the formation of C2 hydrocarbons were almost the same in both catalysts. At CH4/O2 = 1 ratio, the behavior of the 2 and 6 % PbO/SiO2 catalysts differed. There were a lower activity for the oxidation of CH4 to CO2 and a higher activity for the formation of C2 hydrocarbons in the 2 % PbO/SiO2 catalyst than in the 6 % PbO/SiO2 catalyst. At any C H 4 / O 2 ratio, the 10 % PbO/SiO2 catalyst was very different from the 2 and 6 % PbO/SiO2 catalysts. It had a very low activity for the oxidation of CH4 to CO2 and was selective for the generation of C2 hydrocarbons. By doubling the a m o u n t of catalyst, the amount of reacted methane doubled, while the selectivity remained almost constant. Temperature programmed reduction experiments showed almost the same behavior in all the catalysts. However, the 10 % PbO/SiO2 catalyst showed clearly reducible species at about 670 K, which were practically absent in the 2 and 6 % PbO/SiO2 catalysts. Probably, these species were responsible for the low activity for the oxidation of CH4 to CO2 in the 10 % PbO/SiO2 catalyst.

1. INTRODUCTION

The last decade has witnessed great efforts by scientists from many countries to convert methane to value added products. The pioneering work of Keller and Bhasin [1] stimulated great interest in the oxidative coupling of methane. Redox-types oxides constitute a category of catalysts extensively studied for this purpose. Within this category, lead oxide and supported lead oxide

738 have been reported to be suitable for the OCM reaction [2-7]. These studies have demonstrated that supported lead oxide catalysts are very active and that their catalytic behavior depends strongly on the lead oxide loading; however, a clear u n d e r s t a n d i n g has not been reached yet. With respect to the mechanism, Bytin and Baerns [8] distinguished two adsorption steps for m e t h a n e on lead oxide: (a) dissociative adsorption with the s u b s e q u e n t recombination of the adsorbed fragments to yield ethane, and (b} adsorption as methylcarbonium species on acid sites, which then undergo attack by surface O"- ions, yielding methoxide species, which then undergo deep oxidation. In this study, we tried to explain the effect of the PbO loading and the CH4/O2 ratio on the oxidative coupling of m e t h a n e (OCM) over PbO/SiO2 catalysts. Special emphasis was made in the interpretation of the product distribution in the gas phase and its relationship with the n a t u r e of the catalyst surface. The methodology of the investigation involves incorporation of variable a m o u n t s of PbO to the SiO2 to examine the product distribution for both low and high lead oxide loadings. Temperature-programmed reduction (TPR) reveals the surface changes of PbO.

2. EXPERIMENTAL 2.1. Catalyst preparation Pb(NO3)2 (99.101%) and SiO2 (Davisil, grade 646, SB~=245 m2/g) were p u r c h a s e d from Riedel and Fisher, respectively. Before preparation of the catalysts, the silica was calcined at 1200 K for 4 h. PbO/SiO2 catalysts were prepared by impregnating a m o r p h o u s SiO2 (60-80 mesh) with aqueous solutions of Pb(NO3)2 of appropriate concentration to yield Pb0 loadings of 2, 6 and 10 wt. %. Excess water was removed in a rotary evaporator. The catalysts were then dried at 393 K in an oven for 12 h and subsequently calcined at 1073 K for 4 h. 2.2. Reaction s y s t e m Methane conversion was performed in a conventional ruxed-bed continuous flow reactor operated u n d e r atmospheric pressure. The reactor consisted of a quartz, U type tube of 9 m m internal diameter. The a m o u n t of catalyst used for a test r u n was about 0.25 g, which was held in place by quartz wool plugs. The reactor was placed in an electric furnace with approximately 20 cm of the quartz-filled tube serving as a preheater. Before the reaction the catalysts were pretreated in an oxygen flow at 1048 K for 1 h. The reactant mixture of CH4 and 5% 02 in He was adjusted to meet several CH4/O2 ratios and a total flow rate of about 1.2 dm3/h, keeping constant the oxygen

739 partial p r e s s u r e (4.5 kPa). The coupling reaction w a s carried o u t at 1048 K for at least three h o u r s to establish steady state conditions. Catalyst deactivation w a s not observed over this time period. The r e a c t a n t s a n d p r o d u c t s were analyzed with a n o n - s t r e a m gas c h r o m a t o g r a p h equipped with a TCD. Two c o l u m n s , a C h r o m o s o r b 102 (3 m), a n d a Molecular Sieve 5A (2.5 m) were employed in the analyses. Care w a s t a k e n to avoid c o n d e n s a t i o n of the p r o d u c t s at the outlet of the reactor. The conversion a n d selectivities were calculated from the a m o u n t s of reaction p r o d u c t s formed (carbon a t o m basis) as d e t e r m i n e d by the GC analysis. The error in c a r b o n balance w a s found to be below 5% in all cases. Total conversion of r e a c t a n t (XT) a n d selectivity to p r o d u c t i (Si) are defined as;

XT --'--

Si =

moles of reactant transformed X 100 moles of reactant in the feed

C atoms of i 9 moles formed of i x 100 moles of CH4 transformed

2.3. Experimental

techniques

The surface a r e a s of the catalysts were m e a s u r e d by the conventional BET nitrogen a d s o r p t i o n method. Values of 26, 7 a n d 4 m 2 / g were o b t a i n e d for the 2, 6 a n d 10 % PbO/SiO2 catalysts, respectively. T e m p e r a t u r e p r o g r a m m e d reduction e x p e r i m e n t s were performed u s i n g an a p p a r a t u s described by Robertson et al. [9]. The reduction w a s carried o u t with a purified h y d r o g e n - a r g o n mixture (10 vol.% hydrogen) at a h e a t i n g rate = 10 K min -I u p to 1048 K. The TPR reactor w a s charged with 0.25 g of c a l c i n e d (fresh) catalyst. Before the reduction, the catalyst w a s oxidized in a 02 flow to 1048 K for 1 h, a n d t h e n cooled down to 300 K in Ar. This reduction-oxidation cycle w a s repeated several times.

3. RESULTS AND DISCUSSION

Table 1 s h o w s the catalytic properties for the oxidative coupling of m e t h a n e over 2, 6 a n d 10 % PbO/SiO2 catalysts at different CH4/O2 ratios at 1048 K, u s i n g 250 mg of catalyst. It w a s also r u n u s i n g 500 mg of 10 % PbO/SiO2 catalyst at a CH4/O2 = 2 ratio. As it is well k n o w n on OCM catalysts, the CH4 conversion decreases a n d the C2 selectivity increases w h e n the CH4/O2 ratio increases. For the 2 a n d 6 % PbO/SiO2 catalysts,

740 the 02 conversion is less t h a n 100 % only for a CH4/O2 = 1 ratio, while for the 10% PbO/SiO2 catalyst, it is always well below 100 %. Besides, by doubling the a m o u n t of the 10% PbO/SiO2 catalyst, the conversion of m e t h a n e almost doubles, while the selectivity remains nearly constant. However, Table 1 does not provide sufficient information a b o u t the behavior of these catalysts. Therefore it is necessary to look further into the feed and p ro du ct gas composition.

Table 1 C a t a l ~ i c Properties of PbO/SiO2 catalysts PbO load P(CH4) Conv CH4 Conv 02 /P{O2)

(%1

(O/o)

(o/o)

C2H6

Selectivity

(%)

C2H4

CO

CO=

2 (250 mg)

1 2 5

39.3 29.0 15.6

65.4 100 100

7.9 9.9 13.3

11.1 15.8 25.0

5.9 4.8 4.2

75.1 69.5 57.5

6 (250 mg)

1 2 5

48.8 30.4 16.4

86.4 100 100

5.8 13.2 14.3

7.8 17.5 25.4

2.3 4.2 5.3

84.1 65.1 55.0

10 (250 mg)

1 2 5

8.2 6.4 3.6

6.5 7.4 12.0

46.7 53.9 60.7

19.1 22.5 19.7

8.7 4.7 3.6

25.5 19.0 16.0

10 (500 mg)

2

11.3

17.3

44.1

25.6

5.2

25.1

Reaction conditions: F = 1.2 din3/h, P(O2)ffi 4.5 kPa, Ptot~ffi 101 kPa, T = 1048 K, Inert = helium.

Figure 1 shows the c o n s u m p t i o n of CH4 and 02, and the a m o u n t of the different p r o d u c t s formed for several CH4/O2 ratios over a 6 % PbO/SiO2 c a t a l y s t . For a CH4/O2 = 2 ratio, the c o n s u m p t i o n of CH4 reaches a value n e a r or equal to the m a x i m u m a m o u n t allowed by the a m o u n t of oxygen available for the reaction. At this point all of the available oxygen is c o n s u m e d (100% O2 conversionl. For a CH4/O2 = 1 ratio, the c o n s u m p t i o n of CH4 does not reach the m a x i m u m allowed value (86.4 % 02 conversion). This is because with a decreased a m o u n t of m et hane, and a c o n s t a n t

741 a m o u n t of oxygen, competition for the active sites o c c u r s where the m e t h a n e is disfavored, so t h a t the m e t h a n e c o n s u m p t i o n decreases. This competition between m e t h a n e a n d oxygen h a s been noted in the literature [10]. For a CH4/O2 --- 5 ratio, the excess m e t h a n e displaces the oxygen on the active sites. It i n c r e a s e s the m e t h a n e c o n s u m p t i o n until there is no more oxygen available (100% 02 conversion) to regenerate the active sites, resulting in c o n s t a n t a m o u n t of c o n s u m e d m e t h a n e at the steady state. In this situation, the n u m b e r of active sites utilized can be less or equal to the total n u m b e r of active sites. Figure 1 also shows t h a t w h e n m e t h a n e is in excess with respect to oxygen, t h a t is, at higher CH4/O2 ratios, the oxidative coupling of m e t h a n e is favored over total c o m b u s t i o n . It is evidenced by the increasing p r o d u c t i o n of C2 h y d r o c a r b o n s a n d the decreasing a m o u n t of CO2. At these conditions, the m e t h a n e m u s t occupy a higher proportion of the surface, limiting the a m o u n t of oxygen t h a t h a s access to it. This situation favors a methyl radical p r o d u c t i o n a n d their coupling in the gas p h a s e a n d disfavors the oxidation of CH4 to CO2 on the surface. When m e t h a n e is in deficit, t h a t is, CH4/O2 < 1 ratio, the excess surface oxygen restricts the formation of m e t h y l radicals. Rather, m e t h a n e is deeply oxidized, favoring the p r o d u c t i o n of CO2.

o

3

D

2,5

uJ O

--o--02

am

o

-r

n,,o O(9 1,5 a ~

-o-- C02 o----o.-._...._

~-. C2H4 (*)

I--

n,,l~ w > =-0,5 z

o0

C2H6 (*) --

0 0

1

2

3

4

5

CO

6

CH4/O2 RATIO

Figure i. Effect of the CH4/O2 ratio on the gas p h a s e composition in a 6 % PbO/SiO2 catalyst. (*) Equivalent ~mol of CH4 = C atoms of i product. #Jmol formed of i product

742 Figure 2 shows the consumption of CH+ and 02, and the a m o u n t of several p r o d u c t s versus the CH4/O2 ratio for a 2 % PbO/SiO2 catalyst. The behavior of this catalyst is the same as the 6 % PbO/SiO2 catalyst for CH4/O2 > 2 ratio. The activity for the deep oxidation of CH+ to CO2 and the activity for the formation of C2 hydrocarbons are almost the same in both catalysts.

3 uJ

o

2,5

o

2

.--0--02 9--r

CH4

--o--- CO2 x

--b-- C2H6 (*)

0,s

--

Z

o

(3

C2H4 (*)

CO

0 0

1

2

3

4

5

6

CH4/O2 RATIO

Figure 2. Effect of the CH4/O2 ratio on the gas p h a s e composition in a 2 % PbO/SiO2 catalyst. (*) Equivalent pmol of CH, = C atoms of i product, pmol formed of i product For a CH+/O2 = 1 ratio, the behavior in both catalysts differs slightly. There are a lower activity for the oxidation of CH4 to CO2 and a higher activity for the formation of C2 hydrocarbons in the 2 % PbO/SiO2 catalyst t h a n in the 6 % PbO/SiO2 catalyst. For this ratio, the adsorption of oxygen competes with that of methane, and results in a lower m e t h a n e c o n s u m p t i o n t h a n in the case of a CH4/O2 = 2 ratio. The lower consumption of m e t h a n e in the 2 % t h a n in the 6 % PbO/SiO2 catalyst at CH4/O2 = 1 ratio, corresponds to a decrease in the conversion of m e t h a n e to CO2 in the 2 % PbO/SiO2 catalyst, and an increase in the conversion of m e t h a n e to C~ hydrocarbons. The decrease in the production of CO2 a n d the increase in the production of C2 h y d r o c a r b o n s would be related to a lower n u m b e r of active sites for the deep oxidation of m e t h a n e a n d to a higher n u m b e r of active sites for the formation of methyl radicals in the 2 % PbO/Si02 catalyst. Figure 3 shows the consumption of CH4 and 02, and the a m o u n t of p r o d u c t s versus the CH4/02 ratio for the 10 % PbO/SiO2 catalyst. It is

743 observed that the oxygen consumption is very low, so there is excess of oxygen in the gas phase at any time, and oxygen suppy is not limiting the rate. The consumption of methane increases with the a m o u n t of methane in the gas phase. This consumption is strongly related to the production of C2 hydrocarbons and to the active sites able to generate methyl radicals, while the low production of CO2 suggests a shortage of active sites for the oxidation of methane to CO2.

0,5 o LU o :::) am O r,r n,'o 0(.9

uJ~

0,4

---o-- CH4 ---0.-02

0,3

C2H6 (*) 7. C2H4 (*)

0,2

IuJ :~. > 0,1 z O o

-o--CO2 #, CO

0 0

1

2

3

4

5

6

CH4/O2 RATIO

Figure 3. Effect of the CH4/O2 ratio on the gas phase composition in a 10% PbO/SiO2 catalyst. (*) Equivalent #Jmol of CH, = C atoms of i product, pmol formed of i product Evidently the I0 % PbO/SiO2 catalyst has a different nature than the two lower loading s a m p l e s , in the sense that it has something that inhibits the deep oxidation of methane, increasing the generation of methyl radicals and so the production of C2 hydrocarbons. Figure 4(a} compares the production of ethane, ethylene and carbon monoxide from the various catalysts. Figure 4(b) compares the generation of carbon dioxide from these catalysts, and shows that only the 2 % and 6 % PbO/SiO2 catalysts have high production of CO2. Clearly the 10 % PbO/SiO2 catalyst has undergone a modification in its structure, which allows lower formation of CO2.

744

,2

a uJ

o

1,2

......

1

a

--o--- 6 % PbO

o ~ . 0,8 n~

DU)

0,8

- . - o - 2% PbO

0.6

~.~ +

---a-- 10% PbO

o.j

0,4

0,4

0 o "0,2

+"0,2 "1r

|

,

|

|

0 0

1 2 3 4 5 6 CH4102 RATIO

0

(a)

1 2 3 4 5 CH4/O2 RATIO

(b)

Figure 4. Effect of the PbO loading in the OCM reaction over PbO/SiO2 catalysts. (*) Equivalent lJmol of CH4 = C atoms of i product, lJmol formed of i product

Ill 1 I'--

2 O E ia,.

PbO 10%

=E

6%

z

o

2%

O N

:z: 0

400

I

|

I

I

600

800

1000

1200

TEMPERATURE (K)

Figure 5. TPR profiles for PbO/SiO2 catalysts.

6

745 Figure 5 compares the results of the TPR experiments and shows that all the catalysts have similar behavior. However, the 10 % PbO/SiO2 catalyst shows clearly a reduction feature at about 670 K, which is absent in the 2 and 6 % PbO/SiO2 catalysts. The presence of some reducible species may be responsible for the very low activity for the deep oxidation of CH4 to CO2 in the 10 % PbO/SiO2 catalyst.

4. CONCLUSIONS

At CH4/O2 -> 2 ratio, the behavior of the 2 and 6 % PbO/SiO2 catalysts is very similar. The activity for the deep oxidation of CH4 to CO2 and the activity for the formation of methyl radicals ( C2H6, C2H4 and CO) are almost the same in both catalysts. At CH4/O2 = 1 ratio, the behavior of the 2 and 6 % PbO/SiO2 catalysts differs slightly. There is a lower activity for the oxidation of CH4 to CO2 and a higher activity for the formation of C2 hydrocarbons in the 2 % PbO/SiO2 catalyst than in the 6 % PbO/SiO2 catalyst. At any CH4/O2 ratio, the 10 % PbO/SiO2 catalyst has a very low activity for the deep oxidation of CH4 to CO2 and its catalytic activity is for the generation of methyl radicals. By doubling the a m o u n t of catalyst, the a m o u n t of reacted methane doubled, while the selectivity remained almost constant. Temperature programmed reduction experiments show almost the same behavior in all the catalysts. However, the 10 % PbO/SiO2 catalyst shows a reducible species at about 670 K, which is absent in the 2 and 6 % PbO/SiO2 catalysts. Probably, these species are responsible of the very low activity for the oxidation of CH4 to CO2 in the 10 % PbO/SiO2 catalyst.

REFERENCES

I. G.E. Keller and M.M. Bhasin, J. Catal., 73 (1982) 9. 2. W.Hinsen, W. Bytyn and M. Baerns, in Proceedings 8 th International Congress on Catalysis, Berlin, 1984, Vol. 3, Verlag Chemie, Weinheim, 1984, p.581. 3. K. Asami, S. Hashimoto, T. Shikada, K. Fujimoto and H. Tominaga, Ind. Eng. Chem. Res., 26 (1987) 7. 4. G. Wendt, C.D. Meinecke and W. Schmitz, Appl. Catal., 45 (1988) 209. 5. A. Machocki, A. Denis, T. Boroniecki and J. Barcicki, Appl. Catal., 72 ( 1991) 283. 6. S.E. Park and J. S. Chang, Appl. Catal. A 85 (1992] 117,

746 7. R. Mariscal, J. Soria, M. A. Pefia and J. L. G. Fierro, Appl. Catal., A: General 111 (1994) 79. 8. W. Bytyn and M. Baems, Appl. Catal., 28 (1986) 199. 9. S. D. Robertson, B.D. McNicol, J.H. De Bass, S. C. Kloel and J. W. Jenkins, J. Catal., 37 (1975) 424. 10. G.A. Martin and C. Mirodatos, Fuel Processing Technology, 42 (1995) 179.