Methane activation without using oxidants over supported Mo catalysts

~ ELSEVIER

APPLIED CATALYSS I AG : ENERAL

Applied Catalysis A: General 136 (1996) 7-17

Methane activation without using oxidants over supported Mo catalysts She-Tin Wong, Yide Xu *, Wei Liu, Linsheng Wang, Xiexian Guo State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Received 20 April 1995; revised 4 October 1995; accepted 6 October 1995

Abstract

The catalytic properties of Mo/HSAPO-34, Mo/H-ZSM-5 and M o / H Y catalysts for methane conversion without using oxidants were compared. Mo/H-ZSM-5 catalyst has the best performance in terms of catalytic activity and stability. This catalyst is very selective for benzene production and the benzene yield is the highest of the three catalysts studied. The catalytic activity and benzene selectivity over the Mo/HSAPO-34 catalyst, however, decreased with increasing time on stream. As the catalyst deactivated, the selectivity to ethene increased. The M o / H Y catalyst is active for methane conversion but rapid deactivation of the catalyst resulted in negligible product yield. These results are discussed in terms of the shape-selectivity effect of the support taking into consideration the high reaction temperature. Preliminary studies on alkali metal doped Mo/HSAPO-34 catalysts showed no observed effect on catalyst deactivation. Keywords: Methane activation; Shape-selectivity; Alkali metal doping

1. Introduction The chemistry of converting C 1 sources into valuable higher molecular weight hydrocarbons has always been a focal point of catalytic research. We have an abundant supply of methane but it is a very inert molecule and relatively difficult to activate. Hence, progress in this aspect is slow. The most extensively studied methane activation process is the oxidative coupling of methane (OCM) [1]. This process was first reported by Union Carbide in 1982. Most of the catalysts active for this reaction are composed of or modified with alkali and * Corresponding author. Fax. ( + 86-411) 4691570. 0926-860X/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0926-860X(95)00260-X

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S.-T. W(mg et al./Applied Catalysis A: General 136 (1996) 7-17

alkali earth metal compounds. Thus, catalyst basicity is of significant importance for the OCM reaction. Although there has been a great deal of work done in this area since 1982, this route is still far from meeting the conditions required for industrial application, unless the C~- yield can exceed 25%. Recently, we have started the study of methane activation under non-oxidative conditions [2,3]. In our previous works, we have shown the possibility of the selective conversion of methane to aromatics over Mo modified H-ZSM-5 catalysts (Mo/H-ZSM-5). This route is quite similar to homogeneous superacid-assisted methane activation where an oxidant is not necessary [4]. In that case, protonation on C - H bonding of methane occurred to form a two-electron-three-center carbonium ion. However, since the presence of Mo active sites in H-ZSM-5 increases greatly the catalytic activity of methane conversion, it is thus unlikely that protonation is the major mechanism over Mo/H-ZSM-5 catalysts. In addition, we have noted that only transition metals at high oxidation states are active in this type of reaction. Therefore, we proposed that methane activation in this case initiates via heterolytic splitting of the C - H bond to form CH~-, with the assistance of Br0nsted acid sites [3]. As the temperature of pretreatment and reaction are as high as 973 K, some C - H bond splitting by hydride abstraction may be assisted by Lewis acid sites. There are a large variety of solid acids which can be used as catalyst supports for the methane activation reaction. In this study, we have prepared Mo catalysts with three different types of molecular sieves as supports. Special attention is focused on HSAPO-34 with small cage windows which is known to restrict the products of the methanol conversion reaction to lower and linear hydrocarbons [5,6]. Their temperatures of reaction are, of course, lower than that for the methane activation reaction. The influence of the framework structure of these molecular sieves on the catalytic activity and product selectivity of methane conversion is discussed.

2. Experimental 2.1. Materials

Four different types of supports were used in the preparation of transition metal modified catalysts. Zeolites ZSM-5 ( S i O 2 / A I 2 0 3 = 50) and KL were supplied industrially, zeolites Y and SAPO-34 were supplied by this institute. Except KL, all the supports were used in the acid form. Mo and Zn modified catalysts, represented as Me/Support, were prepared by impregnating the support with ammonium heptamolybdate [(NH4)6Mo7024"4H20] and zinc nitrate, respectively. After drying at 383 K, the impregnated sample was calcined at 773 K for 6 h. An alkali metal doped Mo/HSAPO-34 catalyst was prepared by sequential impregnation. Li and K were incorporated into the

S.-T. Wong et al. /Applied Catalysis A: General 136 (1996) 7-17

9

catalyst prepared above in the form of nitrate. After the doped catalyst was dried at 383 K, it was again calcined at 773 K for 6 h.

2.2. Catalytic tests Catalytic tests were performed in a fixed bed quartz microreactor operating in a continuous flow mode. The catalyst was first pelletized without binder, crushed, and sieved to 4 0 - 6 0 mesh. 0.2 g of the catalyst was then packed at the center of the reactor, and was covered with a pre-heated zone of quartz sand. Before the catalytic run, the catalyst was flushed with air at the reaction temperature of 973 K for 40 min. The hourly space velocity of methane was 1500 m l / g h, and the pressure of the reactor system was about 15 kPa. The reactor effluent was analyzed on-line with a Shimadzu GC-9AM gas chromatograph at 40 min intervals. Conversion of methane and product selectivity were calculated on a carbon number basis. Analysis of the air and He used showed the absence of H 2 and hydrocarbon impurities. Methane was 99.95% pure.

3. Results 3.1. Reaction of methane over Mo / HSAPO-34 The results of methane conversion over M o / H S A P O - 3 4 catalysts are presented in Table 1. MOO3, HSAPO-34 and Z n / H S A P O - 3 4 are also included for comparison. The data listed are those at the maximum observed conversion in the reaction. Pure MoO 3 and blank HSAPO-34 are virtually inactive for methane conversion at 973 K, however, M o / H S A P O - 3 4 catalysts are very active. The activity of the M o / H S A P O - 3 4 catalysts approaches a maximum at a Mo loading of

Table 1 Methane conversion over various catalysts Catalyst a

CH 4 conversion (%)

HSAPO-34 MoO 3 Mo/HSAPO-34 Mo/HSAPO-34 b Zn/HSAPO-34

0.01 0.01 0.60 0.61 0.04

Selectivity (%) C2

C2

C3

Benz.

Tol.

31.0 50.9 40.2 33.1 61.1

69.0 49.1 17.5 16.2 38.9

0 0 1.8 1.4 0

0 0 36.4 45.0 0

0 0 4.1 4.2 0

a The metal loading of these HSAPO-34 supported catalysts is 4%. b Regenerated catalyst: under an air flow at 873 K for about 1 h.

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S.-T. Wong et al. /Applied Catalysis A: General 136 (1996) 7-I 7 0.80

100 9O

¢: O

0.64

- ~G . . . . .

o > e-" O

0.48 I

L)

0.32 I

o e¢1 cO

[] . . . . .

[] . . . . .

8O 70

;¢

6O

>,

50

>

40

30

O

®

~

21) 10

0.16 0.00÷

...... + 10

" 55

" ÷

'

+

1 O0

'

. . . .

145

'...... 190

TIME ON STREAM

' 235

"

280

(MIN)

Fig. l. Effect of time on stream on methane conversion reaction over 2% Mo/HSAPO-34 catalyst. (O) Methane conversion, ([]) selectivity for ethene, ( • ) ethane, ( + ) propene, ( A ) benzene.

about 1%, and further increases in loading do not affect the activity significantly. On the other hand, the activities of Zn/HSAPO-34 are negligible. Fig. 1 shows the variation of total activity and product selectivity in methane conversion with time on stream over a typical M o / H S A P O - 3 4 catalyst. The selectivity of toluene with only a datum at 40 min of reaction (4.2%) is omitted for simplicity. At the initial stages of the reaction, the selectivity to aromatics is always high. However, both the total activity and selectivity of the aromatics decreases dramatically with increasing time on stream. As this happened, the selectivity of ethene increases sharply and is >~ 70% after 2 h on stream. The

0,30

0,24

0.18 '1o

0.12

0.06

0.00 10

55

100

145

TIME ON STREAM

190

235

280

(MIN)

Fig. 2. Relationship of benzene-ethene yield over 2% Mo/HSAPO-34 catalyst. (D) Ethene yield, ( ~ ) benzene yield.


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Table 2 Maximum activity data for methane conversion over Mo/HSAPO-34, Mo/H-ZSM-5 and M o / H Y catalysts Catalyst a

Mo/HSAPO-34 Mo/H-ZSM-5 Mo/HY

CH 4 conversion (%)

0.67 6.65 0.01

Yield (%) Ethene

Ethane

Propene

Benzene

Toluene

0.17 0.22 0

0.07 0.16 0.01

0.01 0.01 0

0.39 5.92 0

0.03 0.34 0

" Mo loading: HSAPO-34 (1%), H-ZSM-5 (3%), HY (6%).

selectivities of C2-C 3 hydrocarbons amount to about 90%. In addition, shortly after the sharp decrease in the yield of benzene as the catalyst deactivates, ethene yield increases and reaches a maximum (Fig. 2). Furthermore, increasing the contact time during the reaction leads to an increase in total activity and aromatic (and also ethane) selectivity, while the selectivity to ethene decreases. For example, when the CH 4 conversion is increased from 0.17 to 0.28% by doubling the contact time, the ethane and benzene selectivities increased from 7.9 and 9.7% to 12.6 and 17.3%, respectively, whereas the ethene selectivity decreased from 78.1 to 66.6%.

3.2. Effect of supports Table 2 compares the product yields of methane conversion over Mo/HSAPO-34, Mo/H-ZSM-5 and M o / H Y catalysts. The values at maximum conversion are selected for this comparison. The catalytic activity is the highest over Mo/H-ZSM-5 with maximum yield on benzene. The product yields over Mo/H-ZSM-5, however, do not vary significantly with time on stream. The yield of lower hydrocarbons (C 2 + C 3) over Mo/HSAPO-34 is comparable to that over Mo/H-ZSM-5, but fewer aromatics are produced. For

Table 3 Selectivity data for methane conversion over Mo/HSAPO-34, Mo/H-ZSM-5 and M o / H Y catalysts, and their supports Catalyst

a

HSAPO-34 Mo/HSAPO-34 H-ZSM-5 Mo/H-ZSM-5 a Mo/H-ZSM-5 b HY Mo/HY

CH 4

0.01 0.32 0.05 1.23 6.65 0.01 0.01

conversion (%)

Selectivity (%) b Ethene

Ethane

Propene

Benzene

Toluene

23.4 70.8 20.0 16.4 3.3 0 0

76.6 13.4 60.0 4.9 2.4 100 100

0 4.0 0 0.8 0.2 0 0

0 11.9 20.0 73.0 89.0 0 0

0 0 0 4.9 5.1 0 0

a Mo loading: HSAPO-34 (1%), H-ZSM-5 a (1%), H-ZSM-5 b (3%), HY (6%). b Recorded at 120 rain on stream.

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Table 4 Activity data (CH 4 conversion recorded at 200 min on stream) of methane conversion reaction over Li- and K-doped Mo/HSAPO-34 catalysts Catalyst

KLi-

Ra 0

1.0

2.5

5.0

0.26 0.26

0.28 0.24

0.27 0.22

0.21 0.21

a Mole ratio of alkali metal and Mo (m/Mo).

both cases, the initially white colored catalyst turned black after reaction. Surprisingly, M o / H Y did not perform as well as expected. However, the color of the catalyst turned bluish grey after reaction. As expected, M o / K L is not active and the spent catalyst has the original white colour. Table 3 presents the typical product selectivity patterns of Mo/HSAPO-34, Mo/H-ZSM-5 and M o / H Y at 120 min on stream. The reaction data of the catalyst supports are also included for comparison. It should be pointed out that the product selectivity over Mo/H-ZSM-5 remained quite stable throughout the reaction. However, the selectivity for aromatics over Mo/HSAPO-34 decreases sharply with increasing time on stream, as mentioned above. Since the catalytic performance of these catalysts is very different, it is unwise to compare the product selectivity of these catalysts closely. However, one can still appreciate the high selectivity of benzene and ethene over Mo/H-ZSM-5 and Mo/HSAPO-34 catalysts, respectively.

3.3. Effect of alkali metals Table 4 lists the conversion of methane over Li- and K-doped Mo/HSAPO-34 catalysts at 200 min on stream. The Mo/HSAPO-34 catalyst selected for this study has a Mo loading of 1%. It can be seen that both the activity of the undoped and the doped catalysts are similar, since the variations in methane conversion are within the limit of deviation.

4. Discussion

4.1. Choice of support The choice of support in the present study was based on both structural and chemical considerations. Three types of acidic supports with different pore structures and sizes were selected. These are HSAPO-34 (small pore), zeolite H-ZSM-5 (medium pore) and zeolite HY (large pore) molecular sieves. With these supports, the effect of the framework structure on the reaction process for


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methane conversion can be studied. Additionally, zeolite KL (large pore) which is a basic support was also included in this study for comparison. The structures of the supports selected are briefly described below. Detailed description can be found in the literature [5,7,8]. HSAPO-34 has the framework topology of the natural zeolite chabazite. Its framework structure contains a three-dimensional channel system consisting of large ellipsoidal cages 11 long and 6.5 A wide. Each of these cages can be entered through the 8-membered ring windows with a diameter of about 3.8 A. The framework structure of the zeolite H-ZSM-5 contains a two-dimensional channel system consisting of two sets of intersecting channels, one straight (5.4 X 5.6 A) and the other sinusoidal (5.1 × 5.4 ~,). In the case of zeolite HY, its framework structure contains a three-dimensional pore system consisting of supercages of about 13 ,~ in diameter. These supercages are interlinked through cage windows of about 7.4 A in diameter. The framework structure of zeolite KL contains a unidimensional channel system with pore diameter of about 7.1 o

4.2. Methane conversion reaction

The catalyst supports and M o O 3 showed negligible activity. However, the activity for the non-oxidative methane conversion reaction can be improved by the addition of Mo ions onto the supports. Therefore, it is clear that this conversion process requires a bifunctional catalyst with both Mo ions and Br0nsted acid sites. The Brcnsted acid sites of these supports such as HSAPO-34 are located in the pore systems [9]. However, as the impregnated ammonium heptamolybdate is situated mainly on the external surfaces of the support, calcination of the as-synthesized catalyst will inevitably lead to the presence of Mo active sites both on the external surfaces and in the pore systems. The former Mo active sites were found to contribute much more to the total methane conversion than the latter. Detailed discussion on the role of the Mo active sites and the Brcnsted acid sites on e.g. Mo/H-ZSM-5 catalysts will be the subject of a separate publication. Since the Zn/HSAPO-34 catalyst is not active for methane conversion, it is possible that the ease of methane activation via polarization of the C - H bonds increases with the oxidation state of the transition metal ions. The shape selectivity of molecular sieves on hydrocarbon conversion reactions is a popular topic of research. In particular, zeolite shape-selectivity for methanol conversion has been widely studied [10]. However, the reactions in these studies were usually operated at moderate temperatures ( ~ 673 K). We have noted that both methanol and methane conversion reactions have similar proposed intermediates for aromatic formation, and even the nature of the C 1 species involved in the process of the first C - C bond formation are similar [3,10]. It is thus interesting to see whether similar shape-selectivity effects for

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methanol conversion, particularly product selectivity, also occur for high-temperature methane conversion. Therefore, three of the commonly used catalysts in the methanol conversion study were selected as catalyst supports for this study. Methane conversion over catalysts prepared from these selected supports have shown some unexpected results. Mo/H-ZSM-5 has the best performance in terms of catalytic activity and stability. Unlike in the case of the methanol conversion reaction, Mo/H-ZSM-5 shows high selectivity towards benzene rather than methylated benzene. The product distribution is truncated at C11 (pentamethylbenzene) in methanol conversion, while this happened at Cs (dimethylbenzene) in methane conversion. Thus, alkylated benzene must be unstable at high reaction temperatures where cracking is a predominant process. This suggestion is supported by the absence of aliphatic hydrocarbon products > C 3. However, we cannot omit entirely the effect of pore size reduction by impregnated Mo species. Accordingly, catalyst stability of Mo/H-ZSM-5 is likely due to the effective formation and desorption of benzene formed in the channel system, thereby minimizing the formation of thermodynamically stable polyaromatic coke. This characteristic will indirectly lead to high catalytic activity. An over-simplified thought is that the reactant and product can diffuse in different sets of channels, thereby avoiding counter-diffusion and channel blocking, and finally coking. Anyway, the dual channel system of H-ZSM-5 should be the primary reason for catalytic stability. In methanol conversion over H-ZSM-5 catalyst, however, the resistance to catalyst coking is likely due to shape selectivity effects of the catalyst, by preventing the formation of bulky intermediate coke species, such as polymethylbenzene. HSAPO-34 has a similar acidity to that of H-ZSM-5 [11]. Generally speaking, M o / H S A P O - 3 4 should not be much less active than the Mo/H-ZSM-5 catalyst. In reality, the activity of the former is an order of magnitude lower than the latter. In fact, the yield of lower hydrocarbons over M o / H S A P O - 3 4 is comparable to that of Mo/H-ZSM-5 catalyst. It is thus clear that diffusion restriction occurred in the M o / H S A P O - 3 4 catalysts which lowers the yield of higher hydrocarbons, and therefore the total activity of the catalyst. Unlike the case of methanol conversion over HSAPO-34 where the reaction temperatures were ~< 673 K, the formation of aromatics becomes favourable in the cages at 973 K [5]. However, free diffusion of aromatics out of the cages is restricted by the 8-membered ring windows. Consequently, the increase in occluded aromatics in the cages imposed an additional constraint on the formation and outward diffusion of products. It is interesting to note that as the void volume decreases, the catalyst becomes selective to C 2 - C 3 hydrocarbons. The filling up of the cages with coke (polyaromatic compounds) deactivates the catalyst, and the catalytic activity thus decreases with increasing time on stream. It has been found that diffusion restriction occurs in M o / H S A P O - 3 4 which lowers the total activity of the catalyst. However, it is very surprising to find negligible product yield with the M o / H Y catalyst. This is unlike the case of


15

methanol conversion over the HY catalyst which enables the production of bulkier hydrocarbons than H-ZSM-5. In addition, the dark coloured used catalyst infers that M o / H Y catalyst is active, but the reaction products were retained on the surfaces of the catalyst. As HY is a large pore support with supercages, it should provide the least diffusional limitation on reaction processes. Thus, initiation of the methane conversion reaction will be fast. Accordingly, coke formation from lower alkenes via oligomerization, aromatization and cracking is also a fast process. At high reaction temperatures, the coke formed is highly aromatic or condensed aromatic in nature [12,13]. It is known that pyrolysis (873-923 K) removes the aliphatic chains of coke [14]. Therefore, rapid deactivation of the M o / H Y catalyst occurs through pore or active site blockage by these bulky surface species. It was noted that the activity of methane conversion over M o / H Y catalyst is lower than that of M o / A I 2 0 3 or M o / S i O 2 catalysts [3]. Moreover, it is also lower than that of M o / H S A P O - 3 4 and M o / H - Z S M - 5 catalysts with the same Mo loading. Therefore, the negligible activity of M o / H Y catalyst should not be due to the acidity factor. In general, this finding is consistent with the results from methanol conversion reactions that catalyst deactivation is fairly rapid in small pore and large pore zeolites. The mechanism of methane dehydrogenation and aromatization over M o / H ZSM-5 catalysts has been proposed in our previous study [3]. In this mechanism, heterolytic splitting of the C - H bond is supposed to occur with the formation of CH~-, and finally carbene. These carbenes formed are stabilized by Mo species via the formation of molybdenum-carbene complexes, and thus contribute to the so called C 1 'carbon pool', similar to that proposed by Dahl and Kolboe [6] for hydrocarbon formation from methanol over HSAPO-34. The associated parallel-type mechanism proposed for the formation of lower alkenes, however, cannot explain the high aromatic yield observed in our study, both over M o / H S A P O - 3 4 and Mo/H-ZSM-5. The observed relationship of benzeneethene yield as the M o / H S A P O - 3 4 catalyst decayed showed clearly that ethene is one of the intermediates in the formation of benzene, or aromatics as a whole. This suggestion is also supported by a space velocity study. Therefore, formation of aromatics requires oligomerization of alkenes. In summary, comparing the activity of Mo/HSAPO-34, M o / H - Z S M - 5 and M o / H Y catalysts suggests that the driving force for the methane conversion reaction to occur with high activity and stability is the formation and removal of thermodynamically favoured and stable benzene.

4.3. Alkali metal effect The first impression from our previous study was that alkali metal is detrimental to the methane conversion reaction [3]. In fact, the use of KL as support for this reaction shows that basic supports, which may include alkali

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metal exchanged supports, are not suitable for this reaction. However, the above impression is not entirely true for alkali metal-doped Mo/HSAPO-34 catalysts. For a metal catalyst with acidic support, the effects of alkali metal doping are generally mainly active metal site blocking and acid site neutralization [15]. The latter effect will only be significant if alkali metal is added in sufficiently large amount. In the present study, the amount of alkali metal added is such that not more than 30% of the acid sites are neutralized, in a 1:1 basis. In fact, this percentage will be much less, since only a fraction of the alkali metal added in a sequential impregnation process is involved in acid site neutralization [15]. As we have mentioned before, ethene is the intermediate product of the methane conversion reaction. Therefore, it should be able to prepare a catalyst with a small pore catalyst support which can selectively produce ethene but not the bulkier aromatics. In the case of Mo/HSAPO-34 catalyst, however, the rate of aromatics formation in the cages is very fast and this causes the rapid deactivation of the catalyst. The central idea of this alkali metal doping study is aimed at decreasing slightly the number of acid sites involved in aromatic formation, thereby relieving the overcrowding of the void space and allowing smoother outward diffusion of the product (refer to Section 4.2). However, doping Mo/HSAPO-34 catalyst with alkali metals in this study has no observed effect on slowing down catalyst deactivation. One possibility is that this effect is counterbalanced by the negative effect of Mo active site blocking which lowers the overall activity. It is quite unfortunate that the intrinsic low activity of these catalysts made further detailed studies difficult.

5. Conclusion Based on the results of this study, it was found that different catalyst structures behave differently in methane conversion. The high activity of the Mo/H-ZSM-5 catalyst is due to the ease of formation and removal of thermodynamically stable benzene. Low activity observed with the Mo/HSAPO-34 catalyst is caused by diffusion limitations such as the outward diffusion of aromatics. In the case of the M o / H Y catalyst, the negligible activity is due to the rapid blocking of the pore system and the Mo active sites. It was also found that Mo/HSAPO-34 is a good catalyst for the selective formation of ethene and other smaller hydrocarbons. However, the generally low activity observed for this catalyst has to be greatly improved before it can be applied industrially.

Acknowledgements The financial support of the National Natural Science Foundation of China is gratefully acknowledged.


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