Catalytic properties of yttria doped bismuth oxide ceramics for oxidative coupling of methane

~1~

/ A LE ILYDSS CP AP TA I A: GENERAL

ELSEVIER

Applied Catalysis A: General 159 (1997) 101-117

Catalytic properties of yttria doped bismuth oxide ceramics for oxidative coupling of methane Y. Zeng, Y.S. Lin* Department of Chemical Engineering, University of Cincinnati, Cincinnati, OH 45221-0171. USA

Received 28 August 1996; received in revised form 12 December 1996; accepted 18 December 1996

Abstract

Fluorite structured bismuth oxide-based ceramics are potential membrane materials for oxygen separation and membrane reactor applications. In this work, the catalytic properties of the surface of several bismuth oxide-based ceramics for oxidative coupling of methane were studied using a conventional tubular reactor operated in the cofeed mode. A most commonly studied catalyst, 5 wt% Li/MgO (Li/MgO), was selected as a reference catalyst. 30 mol% Y203 doped Bi203 (BY30) exhibits Cz yield (18%) and selectivity (50%) similar to Li/MgO at 800-850°C, but the former has a C2 space-time yield 15 times higher than the latter. Like Li/MgO, BY30 also yields more C2H4 and CO2 than CEH6 and CO in its products. The comparison studies on Bi203, BY25, BY30 and Y203 show that doping yttria in Bi203 results in an increased C2 yield and a slightly decreased C2 selectivity. A decrease in the surface area of the bismuth oxide-based ceramics gives a larger C: selectivity and a lower C2 yield. Keywords: Oxidative coupling of methane: Yttrium stabilized bismuth oxide; Ionic conduction ceramics; Ceramic membranes

1. Introduction Much work was reported in the past decade on catalytically oxidative coupling of methane (OCM), a promising process for direct conversion of natural gas into C2 (C2H4 and C2H6) products [1-4]. Due to deep oxidation reactions in the gas phase as well as on the catalyst surface, the C2 yield of OCM achieved so far on any catalysts operated in conventional packed-bed reactor is less than 25% when C2 selectivity is higher than 50%, far below the requirement for making OCM economically attractive (>30-40%) [4]. In order to improve the C2 yield, some * Corresponding author. 0926-860X/97/$17.00 ,~ 1997 Elsevier Science B.V. All rights reserved. Pll S0926-860X(97)00009-4

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researchers have recently shifted their focus from searching for better catalysts to developing new types of reactors [5-9]. Due to their several unique properties, ceramic membrane reactors offer potential for improving the C2 yield by genetically minimizing the deep oxidation reactions [10-17]. Most applications of membrane reactors permit the integration of the reaction and separation steps and increase of the yield beyond nominal equilibrium values by selectively removing the desired products from reaction medium. Different from these applications, the OCM membrane reactors are employed to distribute or transport oxygen into reaction medium in a well-controlled manner in order to maintain an optimum oxygen to methane ratio in the reactor. Among different types of ceramic membrane reactors, those made of dense ionic-conducting ceramics appear to be most promising for obtaining higher C2 yield. Lin and his co-workers [18,19] recently pointed out that OCM catalytic properties of the membrane materials are critical to the success of an OCM dense membrane reactor. Three well-known groups of oxygen semipermeable ionic/ mixed-conducting inorganic materials are zirconia-based, bismuth oxide-based and perovskite-type ceramics [20,21 ]. It is known that zirconia-based ceramics are not good catalysts for OCM unless they are promoted with alkali metal compounds (e.g., Na+-ZrO2--C1 -) [22]. The OCM properties of two highly oxygen semipermeable perovskite-type ceramics, Lao.2Sr0.8CoO3 and SrCo0.8Fe0.203, were recently studied in our group [19] by using both steady-state (cofeed mode) and unsteady-state (cyclic mode) methods. In the steady-state study, Lao.2Sro.8CoO3 exhibited good OCM catalytic properties in terms of C2 yield (>14%), selectivity (>50%) and space-time yield (>5 ~tmol/g s) while Sreo0.sFeo.203 showed very poor OCM catalytic properties. However, the C2 selectivity of Lao.2Sro.8CoO 3 decreased substantially (<30%) when operated in cyclic mode with a reducing environment similar to that in a dense membrane reactor. This suggests that Lao.2Sro.8CoO3 may not possess desired OCM catalytic properties when used in an OCM dense membrane reactor. OCM properties of Bi203 (supported on alumina) were first investigated by Keller and Bhasin [23]. They found that the supported Bi203 was not selective to the C2 hydrocarbon formation reactions. This was probably due to the negative effect of the reactor material, stainless steel, on OCM reactions. However, recent studies showed very high C2 selectivity and low C2 yield on Bi-based catalysts [24]. A C2 selectivity of 78% over a Bi203 catalyst was found by Otsuka et al. [25]. It was also known that doping a proper metal into Bi203 would improve its C2 yield. Doped Bi203 catalysts previously studied include: Mg/Bi203 [26], Mn/ Bi203 [27,28], BaLa/BizO3, Y/Bi203 [29], Li/Bi203, Sm/Bi203, Er/Bi203 [30]. Results of OCM on several Bi203 containing catalysts are summarized in Table 1. As shown, some of them exhibit rather good OCM catalytic properties. However, none of the above OCM studies on Bi203-based catalysts were conducted with the intention to select membrane materials for OCM dense membrane reactors. Pure Bi203 is oxygen semipermeable when present in the

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Y Zeng, YS. Lin/Applied Catalysis A: General 159 (1997) 101-117 Table 1 Summary of the OCM properties of some bismuth-based catalysts Catalysts

Temperature

Structure

C2 yield

C2 selectivity

Reference

5% Mg]Bi203 67% Mn]B203

800 730

14 14.4

39.4 34.7

[26] [27,28]

BazLaBiO6 Yo sBi1503 6.3% Li/Bi203 5% Sm/Bi203 1.5% Er/Bi203

800 800 780 780 780

o~-Bi203 Bi2Mn4Olo Trace c~-Bi203 Cubic Cubic ~-Bi203 oz-Bi203 a-Bi203

12.5 12.9 ----

49.9 47.3 65% 93% 47%

[29] [29] [30] [30] [30]

6-phase (fluorite f.c.c structure) which contains a considerable amount of 0 2 vacancy sites and is thermodynamically stable above 740°C [31,32]. The 6-phase Bi203 could be stabilized to even lower temperature when doped with yttrium or erbium [31]. However, the Bi203 in the previous studies was either as an active species coated on a porous support or in the a-phase instead of the 6-phase. Essentially all the experiments were conducted at temperatures below 800°C. The present paper reports OCM catalytic properties of the 6-phase yttria doped Bi203 ceramics in the temperature range 700--900°C. One of the well-known OCM catalysts, Li/MgO, was also studied in this work for comparison purposes. The major objective of this work was to investigate OCM catalytic properties of oxygen semipermeable bismuth oxide ceramics to evaluate the suitability of this material in terms of surface catalytic properties for OCM membrane reactor applications.

2. Experimental 5 wt% Li/MgO samples were prepared from MgO (99%, Aldrich) and LiCO3 (99%, Aldrich) by a wet impregnation method reported in the literature [33,34]. The procedure was detailed elsewhere [19]. Pure Y203 powders were prepared by decomposing Y(NO3)3.6H20 (99.9%, Johnson Matthey) and calcining at 800°C for 10 h. The resulting powders were white and porous. Powder samples of pure Bi203 and yttria doped bismuth oxide (abbreviated as BY) were synthesized by coprecipitation method. The procedure started with dissolving given amounts of Bi(NO3)3-5H20 (99.9%, Fisher) and Y(NO3)3.6H20 in a dilute nitric acid solution. White solid particles were formed and precipitated after adding oxalic acid (99%, Fisher). The precipitates were collected by filtration and dried at 120°C overnight. The resulting powder samples were then calcined sequentially at 400°C, 600°C, and 900°C for 5 h, respectively. The BY catalysts thus obtained are referred to as the powder samples in this paper. Pellet BY samples were prepared from dense BY disks fabricated from the powder samples calcined at 600°C. The procedure for fabricating the BY disks

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included pressing the powder samples in a stainless steel mold under a certain hydraulic pressure, and sintering the green-body disks at a high temperature (>800°C) for a period of 5-20 h. The resulting BY disks had a relative density of 80%. The BY pellet samples were prepared by crushing the BY disks. The crushed BY pellets were further ground to fine particles and the resulting samples are referred to as the particle sample in this paper. Pure Bi203 particle samples were prepared by grinding a yellow piece of pure Bi203 which was obtained by melting the Bi203 powder at 850°C. The surface areas of powder samples were analyzed by nitrogen adsorption using the BET method (Micromeritics ASAP-2000), while those of BY particle and pellet samples were estimated from the particle size determined by using a light microscope. The real surface areas of BY particle and pellet must be larger than the estimated values, since these samples were not sintered to 100% relative density. However, it was found that their surface areas were too small to be measured by the BET method. Thus, the estimated outer surface areas of BY particle and pellet are used to qualitatively indicate the surface area difference among the three BY samples. The phase structure of the samples was examined by the X-ray diffraction (Siemens Kristalloflex D500 diffractometer, with Cu K~, radiation). The OCM catalytic properties were studied in a packed-bed reactor made of a dense alumina (99.8%) tube (1/4 in. i.d., 22 in. long). Methane (99.9%), oxygen (99.9%) and helium (99.9%) (all from Matheson Gas Inc.) were used as the feed gases. A small amount (<1 g) of catalyst was packed in the middle portion of the alumina tube and supported by quartz particles (0.5 mm diameter) from the two sides. Packing density was obtained from the weight of sample packed and its corresponding volume. Bed void fraction was calculated from the packing and real densities of sample (measured by the Archimedes method). Linear velocity and actual contact time of the reactant mixtures were calculated from the reactor volume, bed void fraction and volumetric flow rate (at the actual reaction temperature). The compositions of the feed and product gases were analyzed by a gas chromatography using a Carbosphere column (10 ft long, 1/8 in. diameter, with 80/100 mesh packing materials, Alltech Inc.). In this study the CHJO2 ratio in the feed was fixed at 2.9. The methane conversion is defined as the percentage of methane converted to products and calculated by C =

(QinXCH4 -- aoutYcH4)/ainXCH4,

(1)

where Q stands for the volumetric flow rate, X and Yare molar fractions in the feed and effluent, respectively. The selectivity for the carbon containing product i is the percentage of reacted methane that forms product i and calculated by

Si

=

(aoutniYi) / (ainXcH4 -- aoutYcH4),

(2)

where n; is the number of carbon atoms in the molecule of the carbon containing

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105

product i. The C 2 yield is the percentage of methane that forms ethane and ethylene, which can be calculated from the product of methane conversion and C2 selectivity. The difference of carbon mass balance between the inlet and outlet was found to be less than 5%. In the temperature range investigated the results of OCM in the alumina tube packed with quartz only show less than 2% yield for ethane and carbon oxides and no ethylene in the effluent. This indicates that quartz packed in the alumina tube is catalytically inert to OCM, and that non-catalytic reactions in the empty space of the quartz-packed reactor tube contribute negligibly to the OCM results.

3. Results and discussion XRD analysis shows that all the BY and Bi203 samples prepared in this work have fluorite f.c.c structure, and Y203 sample has a body-centered cubic structure. XRD patterns of a 30 mol% yttria doped Bi203 sample (abbreviated as BY30) and 5 wt% LifMgO sample (abbreviated as Li/MgO) are shown in Fig. 1. The XRD peaks for the Li/MgO sample are mainly of cubic MgO with trace amount of Li20 phase, indicating that solid solution is not formed. Table 2 summarizes the preparation methods, reaction conditions and OCM catalytic properties for BY30, BY25 (25 mol% Y203/Bi203), pure bismuth oxide, pure yttria and Li/ MgO samples for which OCM properties were measured. The two BY samples were prepared with yttria in such a concentration range because bismuth oxide could be stabilized in the &phase with yttria content of 22.5-32.5 mol% [32]. As shown, the surface areas of Bi203-based samples are very small (<1 m2/g), indicating that the samples contain relatively large dense particles. The preparation method and XRD data show that all BY30 and pure Bi203 samples are in the form of solid solution and Li/MgO is more likely in a structure consisting of a Li20 layer coated on the surface of MgO particles. It should be pointed out that the temperature range (700-900°C) of interest to this work is higher than most other work on OCM catalysts. This is because oxygen permeability of BY membranes is not sufficiently high at temperatures lower than 800°C. The effects of flow rate (or space-time) on C2 selectivity and C2 yield of OCM on the materials were studied to identify the optimum flow rate at which the C2 yield was maximum. Fig. 2 shows the flow rate dependence of C2 selectivity and yield of Li/MgO at 800°C and BY30 at 850°C. As shown, C2 selectivity generally increases with increasing flow rate and levels off at 20 ml(STP)/min for Li/MgO or 75 ml(STP)/min for BY30. For C2 yield, however, it decreases with increasing flow rate after reaching a maximum value of 8 ml(STP)/min for Li/MgO or 48 ml(STP)/ rain for BY30. This is due to the fact that the effect of flow rate on methane conversion is opposite to that on C2 selectivity. The optimum flow rates for all studied catalysts are listed in Table 2. Other specific conditions and corresponding

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BY30

_2

Li/MgO I

20

I

30

1

I

i

40

I

50

I

I

I

60

70

2O Fig. 1. XRD patterns of 5% Li/MgO and BY30.

OCM property data are also summarized in Table 2. As shown, the optimum flow rate for BY30 is much higher than that for Li/MgO while the former has a smaller load than the latter. This reflects a higher OCM activity of BY than that of Li/MgO. This will be discussed further in terms of C2 space-time yield. Among the five catalysts summarized in Table 2, Bi203 has the highest C2 selectivity and the lowest C2 yield. The C2 selectivities and yields of BY30 and BY25 are similar to those of Li/MgO, and higher than those of Y203. Figs. 3 and 4 compare the Ce yield and selectivity of BY30 with those of Li/ MgO as a function of temperature (other conditions are listed in Table 2). As shown in these figures, the optimum temperature for both catalysts is different. Li/ MgO requires a lower operation temperature range (700-800°C), in which, the C2 selectivity slightly decreases while C2 yield increases with increasing temperature. This result is consistent with other reports [33,34]. The temperature dependence of the C2 selectivity of BY30 is very different from that of Li/MgO. It increases steadily and, after reaching a maximum of 46% at 800°C, decreases with

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107

Table 2 Characteristics and OCM Properties of Bi203, BY25, BY30, YzO 3 and Li/MgO (CH4 : O2 : He=2.9 : 1 : 1.3) Material formula Bi203

25 mol% Y203/Bi2Oa (BY25)

30 mol% Y203/Bi203 (BY30)

Y203

5 wt% Li/MgO (Li/MgO)

Preparation method Phase structure

Precipitation

Coprecipitation

Coprecipitation

Decomposition

Impregnation

f.c.c, 0.2

f.c.c solid solution 0.6

Cubic

Surface area (mZ/g)a

f.c.c, solid solution 0.5

6.8

Li20 coated MgO crystals 5.1

Amount packed in reactor (g) Flow rate (ml(STP)/min) Reaction temp. CC) C2 yield (%) C2 Spacetime yield (~tmol/g s) C2 selectivity (%)

0.6

0.6

0.1

0.4

0.3

99

99

48

99

8

870

830

850

750

800

4.5 2.76

17 5.48

18 16.9

13.5 6.2

19.3 1.1

60

52

46

43

52

'~BET surface area measured by nitrogen adsorption using a Micromeritics ASAP-2000

60-m

40o oJ

20-

0 0

50 100 150 Flow Rate (ml(STP)/min)

200

Fig. 2. Ca yield and selectivity of Li/MgO as function of feed gas flow rate at 800~C.

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Y. Zeng, Y.S. Lin/Applied Catalysis A." General 159 (1997) 101-117

20

10

O

[

r

700

f

Li/MgO BY30

J

I

800

900

T e m p e r a t u r e ( °C ) Fig. 3. Comparison of C 2 yield of 5% Li/MgO and BY30.

60

.~

r~

~

40

20 Q 0

f

700

i

J

BY30 i

I

800 900 T e m p e r a t u r e ( °C )

Fig. 4. Comparison of C2 selectivity of 5% Li/MgO and BY30.

Y Zeng, YS. Lin/Applied Catalysis A: General 159 (1997) 101-117

109

80

,-,

60

+

40

~

20

/ A

Li/MgO

C)

BY30

0 700

800 900 Temperature ( °C )

Fig. 5. Comparison of percentage of C2H4 in total C2 products for 5% Li/MgO and BY30.

increasing temperature. The temperature dependence of C 2 yield of BY30 follows the same trend as that of C2 selectivity. But the maximum temperature for the C2 yield is about 850°C, slightly higher than that for C2 selectivity due to the increase of methane conversion at high temperature. The best C2 yield and selectivity are 19.3% and 57% for Li/MgO and 17% and 46% for BY30, respectively. The C2 selectivity and yield were measured with both increasing and decreasing temperature. Their differences were less than 5%, indicating that these data are not a function of temperature path. Figs. 5 and 6 compare the percentage of C2H4 and CO over the corresponding total C2 or COx products for Li/MgO and BY30 at different temperatures. Fig. 5 shows that for Li/MgO C2H4 percentage gradually increases with increasing temperature, and reaches 80% at 800°C. Somewhat different trend for C2H4 percentage change with temperature is observed for BY30. There is essentially no C2H4 formed below 780°C. From 780°C to 800°C, C2H4 percentage increases sharply to 60%. Above 800°C, C2H4 percentage further increases gradually to 84% at 900°C. The CO percentage for Li/MgO increases and, after reaching a maximum at 750°C, decreases with increasing temperature. For BY30, the CO percentage, starting at about 0%, increases sharply to 5% as temperature changes from 780°C to 800°C. It reaches 12% at 900°C. The CO percentage for both catalysts in the range 700-900°C is lower than 16%. The C2 space-time, defined as the total amount of C2 (in ktmol) produced per gram catalyst per second, is plotted in Fig. 7 for Li/MgO and BY30 at different

~MgO

Y Zeng, YS. Lin/Applied Catalysis A: General 159 (1997) 101-117

20

,--,"*'~"'~

15

10 0 5

0 I

700

i

I

i

P

800 900 Temperature ( °C )

Fig. 6. Comparison of percentage of CO in total C1 products for 5% Li/MgO and BY30

20

15

10 ~D gl, ¢',1

0

~----~ L'l/MgOr 700

Fig. 7.

C2

800 900 Temperature ( °C )

space-time yield (C2 formation rate) of OCM in 5% Li/MgO and BY30.

Y. Zeng, Y.S. Lin/Applied Catalysis A: General 159 (1997) 101-117

~/~Bi203

60--

@2

so-

.-~

111

40-

¢q

30-

20 700

800

900

Temperature ( °C ) Fig. 8. C2 selectivity of OCM as a function of temperature over bismuth-based oxides with different composition.

temperatures. In this case BY30 has a much higher C2 space-time yield (16.8 ktmol/g s at 850°C) than Li/MgO (1.1 ~tmol/g s at 800°C), indicating that BY30 is much more active than Li/MgO for OCM. For Li/MgO the C2 space-time yield does not increase too much with increasing temperature, while for BY30 it increases dramatically with increasing temperature when lower than 850°C. Again, a jump of C2 space-time yield for BY30 is observed at temperature around 800°C. The OCM catalytic properties on bismuth oxide-based ceramics could be better understood by comparing the results of OCM on pure Y203, BY30, BY25 and pure Bi203. The OCM catalytic properties on the pure Y203, BY30, BY25 and pure Bi203 are compared in Figs. 8 and 9. As shown in Fig. 8, the C2 selectivity for Y203 decreases and that for Bi203 increases with increasing temperature. At temperatures higher than 800°C, the C2 selectivity is the highest for Y203 and the lowest for Bi203. For both BY25 and BY30, C2 selectivity increases and, after reaching a maximum, decreases with increasing temperature. The highest C2 selectivity for BY25 and BY30 is 53% (at 830°C) and 46% (at 800°C), respectively. The C2 selectivity of BY decreases with increasing yttria content. The optimum temperature corresponding to the highest C2 selectivity decreases with increasing yttria content. The results indicate that the oxygen sites related to bismuth are primarily responsible for the C 2 selectivity of OCM. Fig. 9 shows the effect of temperature on the C2 yield over Y203, BY30, BY25 and Bi203. Among the four catalysts, the C2 yield is lowest for Bi203, indicating its

112

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20

15 BY30

\

10

Y203

e,1

Bi203

700

I 800 9O0 Temperature ( °C )

Fig. 9. C2 yield of OCM as a function of temperature over bismuth-based oxides with different composition.

lower activity for activation of methane. Y203, though with a low C2 selectivity, shows a relatively higher C2 yield (10-13.5%). The C2 yield for BY25 and BY30 is higher than those for the pure Bi203 and Y203. This indicates that doping yttria in bismuth oxide greatly improves its OCM activity. For both BY25 and BY30, the C2 yield increases sharply with temperature around 800°C. It is generally agreed that the catalytic oxidative coupling of methane undergoes the dissociation of the methane into methyl radical at the active sites on the catalyst surface. These active sites are usually reactive oxygen species (e.g., O-, O~-) and lattice oxygen species (02-). For alkali promoted alkaline earth oxide catalysts, like Li/MgO catalyst, the reactive oxygen species such as O- or 02- ions on the surface are effective in abstracting hydrogen atoms from methane molecule to form CH3. radicals [33]. In the case of Li/MgO, the activity increases with temperature. Fig. 4 also shows that the C2 selectivity of this catalyst decreases with increasing temperature when operated at the same flow rate. It suggests that the optimum contact time of reactants over Li/MgO in terms of C2 selectivity decreases with increasing temperature. Unlike Li/MgO catalyst, a previous study [35] showed that BizO3 was ineffective in generating gas phase methyl radicals, but selective for formation of Ca product, and the C2 selectivity of OCM on bismuth containing catalysts mainly came from bismuth oxide. Anderson and coworkers [36] suggested that bismuth oxide may

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react with methane via Bi low-lying empty 6s, 6p, 6d band orbitals which result from the partial coordination of bismuth at the surface. According to their theoretical results, as methane homolytically dissociates hydrogen atom is bound to 0 2 - and methyl radical is bound to Bi 3+. High barriers to hydrocarbon radical migration on the surface of o~-Bi203 suggested that CH3. dimerization will occur in the gas phase. Since the BY catalysts studied in this work are ionic conducting ceramics, the OCM properties on these BY catalysts must be also related to electric conduction mechanisms and electron and ion mobility. Zhang et al. [37] pointed out that p-type electronic conduction is an essential requirement for a good OCM ceramic catalyst. Experimental data on electronic conductivity versus oxygen partial pressure on BY30 and BY25 show that the lower limit of oxygen partial pressure for p-type electronic conduction in BY ceramics increases with decreasing temperature and increasing yttria content [31]. The lower limit of oxygen partial pressure for p-type electronic conduction of the BY ceramics at 900°C, estimated from the reported experimental data [31], is 10 -21 atm. In the present study, the oxygen partial pressure in the reactor, estimated from the concentrations of CO and CO2 in the reactor effluent stream using thermodynamic equilibrium data, is larger than 10-15 atm. Therefore, BY catalysts should be of the p-type under the OCM reaction atmosphere in the present experiments. In addition to the electronic conduction mechanism, the oxygen ionic conductivity is also expected to have some effect on the OCM catalytic properties of the BY ceramics. It was shown that the mobility of the non-stoichiometric oxygen in the BY ceramics increased dramatically with temperature around 800°C, and decreased with increasing yttria content [32,38,39]. The strong temperature dependence of the C2 yield and selectivity, C2H4/C2 and C o / c o y ratios at temperatures around 800°C for the BY catalysts studied is consistent with the temperature dependence of ionic conductivity of the BY ceramics in that temperature range [32,36]. Furthermore, the C2 selectivity for BY ceramics decreases with increasing yttria content, which is also consistent with the dependence of ionic conductivity on the yttria content in the BY ceramics. Such consistence between the OCM properties and ionic conductivity indicates that the nonstoichiometric oxygen plays an important role in the formation of C2 hydrocarbons. Inorganic membranes, in tubular or disk shape, usually have very small surface areas because of the constrain in geometric configuration. Therefore, the effect of surface area on the catalytic performance for membrane material is very important. In order to investigate this effect, OCM on three BY25 catalysts with different surface area (BY25 powder, BY25 particle and BY25 pellet) were studied. The characteristics of these three BY25 samples and the results of OCM in a particular temperature are summarized in Table 3. As shown, there is a great difference in the surface area among the powder, particle and pellet samples. Fig. 10 shows the effect of temperature on C2 selectivity on the three BY25 samples. The C2

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Table 3 Characteristics and OCM properties of BY25 catalysts (CH4 : 02 : He=2.9 : 1 : 1.3) Catalyst

BY25 powder

BY25 particle

BY25 pellet

Calcining temp. (°C) Calcining time (h) Particle size Surface area(m2/g) Phase structure Amount packed (g) Flow rate (ml(STP)/min) Temperature (°C) C2 selectivity (%) C2 yield (%) Packing density (g/cm 3) Bed void fraction Liner velocity (crn/s) Contact time (s)

900 5 >200 mesh 0.5 f.c.c., 6-Bi203 0.6 99.0 830 52 17 2.3 0.38 56.8 0.016

850 10 100-150 mesh <0.05 f.c.c.. 6-Bi203 0.6 54.4 830 58 9 3.4 0.47 25.3 0.025

850 10 1-3 nun <0.001 f.c.c., 6-BieO3 0.6 99.0 900 60 11 3.0 0.53 43.3 0.017

60

so o

r~

4o r

20

,

-750

?

i 800

850

900

950

T e m p e r a t u r e ( OC ) Fig. 10. C2 selectivity of OCM as a function of temperature over BY25 with different surface area.

selectivity is the highest for the pellet sample (around 60% at 870°C) and the lowest for the powder sample. This indicates that decreasing surface area has a positive effect on improving C2 selectivity for BY25. The optimum temperature corresponding to the highest selectivity appears to increase with decreasing surface area, from 830°C for the powder sample to 870°C for the pellet sample.

Y Zeng, Y.S. Lin/Applied Catalysis A: General 159 (1997) 101-117

115

20-BY30 Powder 15

BY25 Pellet

10

BY25 Particle

5

I 750

'

I 800

'

I 850

Temperature

'

[ 900

' 950

( °C )

Fig. 11. C 2 yield of OCM as a function of temperature over BY25 with different surface area.

The C 2 yield for the three BY25 samples is plotted as function of temperature in Fig. 11. As expected, at the same temperature C2 yield decreases with decreasing surface area. Unlike the BY25 powder, the C2 yields of both particle and pellet samples always increase with increasing temperature within the investigated temperature range. Nevertheless, the extent of the decrease of C2 yield from powder sample to pellet sample, i.e., from 17% to 11%, is not so significant compared to the extent of the decrease of surface area from powder sample to pellet sample, i.e., from 0.5 m2/g to less than 10 -3 mZ/g. For heterogeneous/homogenous partial oxidative reactions, high surface area of the catalyst is not required. Most reported OCM oxide catalysts have the surface area ranging from 0.1 to 10 mZ/g. However, there is no report on the study of OCM catalyst with the surface area smaller than 0.1 mZ/g, like the BY25 particle and BY25 pellet sample in the present work. The increase in C2 selectivity with decreasing surface area for the BY25 catalysts is consistent with the general rules for OCM reactions because the generated C2 products or methyl radicals are more difficult to be trapped in the active sites of the catalyst surface resulting in further oxidation. Appreciable reactivity of BY25 still remains after the surface area of BY25 decreases by more

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than two orders of magnitude. This is probably related to the high concentration of non-stoichiometric oxygen in the bulk phase of the BY25 catalysts. These experimental results suggest that the small membrane surface area in a membrane reactor may not be a serious problem for OCM reactivity. The packing density, bed void fraction, linear velocity and actual contact time for BY25 powder, particle and pellet are also given in Table 3. The contact time for the powder sample is shortest (highest linear velocity). This is because the powder samples has a porosity (50%) much larger than the particle or pellet samples, and the shorter contact time is required to avoid complete oxidation reactions in its micro- or macro-pores. The pellet sample also requires a shorter contact time in order to minimize the gas phase reactions because of its larger bed void fraction and higher reaction temperature, The above results and discussion show promising features of yttria stabilized bismuth oxide for use as the membrane material for OCM membrane reactor. These results have promoted a further study to examine the OCM properties of this material under conditions similar to those of membrane reactor. Following the methodology developed to examine the suitability of perovskite-type ceramic materials for OCM membrane reactor application in terms of the catalytic properties of the materials [19], the OCM properties of the BY materials in the reducing atmosphere were subsequently investigated using a packed-bed reactor (in unsteady-state cyclic mode) and an electronic microbalance (transient TGA study). The results of this further study are reported elsewhere [40].

4. Conclusions

The surface of yttria doped bismuth oxide ceramics exhibits fairly good catalytic properties for OCM, with C2 yield (18%) and selectivity (46%) comparable to, and C2 space-time yield (16.8 ~tmol/g s) much larger than those for Li/MgO. Temperature has very strong effect on the catalytic performance of BY ceramics. The sharp increase of C2 selectivity and yield at temperature around 800°C is consistent with the dramatic increase of ionic conductivity (non-stoichiometric oxygen mobility) around this temperature. This suggests a strong relationship between OCM catalytic properties of BY ceramics and their ionic conductivity. Yttrium dopant and bismuth oxide seem to play different roles on the catalytic properties of BY ceramics. Yttrium dopant enhances the reactivity of the catalysts, while bismuth improves the Ca selectivity of the catalysts. The optimum operation temperature also decreases with increasing amount of doped yttria. C2 selectivity of BY catalysts is also greatly affected by the surface area of the catalysts. The dense BY25 pellet crushed from sintered disk (1-3 mm in particle size and <10 -3 m2/g in surface area) exhibits the best C2 selectivity (60%) and appreciable Ca yield (12%). The yttria doped bismuth oxide ceramics appear to be the suitable material for membrane reactor for OCM in terms of their catalytic properties.

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Acknowledgements This project was supported by the National Science Foundation (CTS-9502437, Career Award).

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