- Email: [email protected]
Role of chlorine in improving selectivity in the oxidative coupling of methane to ethylene
Applied Catalysis, 46 (1989) 69-87 Elsevier Science Publishers B.V., Amsterdam -
69
Printed in The Netherlands
Role of Chlorine in Improving Selectivity in the Oxidative Coupling of Methane to Ethylene R. BURCH*, G.D. SQUIRE and S.C. TSANG Catalysis Research Group, Chemistry Department, University of Reading, Whiteknights, P.O. Box 224, Reading RG6 2AD (U.K.) (Received 17 June 1988, revised manuscript received 2 September 1988)
ABSTRACT Samarium, magnesium and manganese oxide and alkali-promoted oxide catalysts have been prepared and tested for the oxidative coupling of methane. The results show that alkali-promoted oxides inhibit total oxidation and have a higher selectivity for the formation of C, products than the undoped metal oxides. These catalysts have been promoted by injecting pulses of gaseous chlorinated compounds (dichloromethane and chloroform) during the reaction. It has been found that these chlorinated compounds markedly increase the selectivity for the formation of C, products for all the MnOz-based catalysts and for lithium-doped MgO and SmLzOs catalysts. The effect is greatest in MnO*-based catalysts. When dichloromethane is added to a pure, unpromoted MnOz catalyst the selectivity for the formation of carbon dioxide decreases from 82.6% to 4.1% and the selectivity for the formation of C2H, increases from virtually zero to 56.3%. The highest C, selectivity observed after promotion of pure MnO, by dichloromethane is about 93%. Promotion of these pure oxide catalysts by gaseous chlorinated compounds provides an alternative to alkali promotion as a method of inhibiting total oxidation and of increasing ethylene production.
INTRODUCTION
The pioneering work of Keller and Bhasin [ 1 ] has stimulated interest in the oxidative coupling of methane. This is now recognised as an important first step in the synthesis of higher value hydrocarbons and has the potential of becoming an important industrial process [ 2 1. The fundamental requirement of the oxidative coupling reaction is to inhibit the thermodynamically favourable total oxidation to carbon oxides and enhance the selectivity to higher hydrocarbons principly ethane and ethylene. Keller and Bhasin [ 1]used a technique where methane and oxygen where fed over the catalyst alternately, so that it became correspondingly reduced or oxidised. This alternate feeding or switching of reactants was also adopted by Jones et al. [ 31 who claimed that it offered an increase in C, selectivity without using oxygen directly as the oxidant. However, many research workers use a different method of increasing selec-
0166-9834/89/$03.50
0 1989 Elsevier Science Publishers B.V.
70
tivity to C2 compounds. The reaction is carried out by co-feeding reactants over catalysts which are generally heavily doped with alkali or alkaline earth metal salts [ 4-291. Most work has been concentrated on the effect of different alkali metal dopants without considering the possible influence of the anions used to introduce the alkali metal. However, it has been reported recently that the highest selectivity to ethylene is achieved when chloride ions are present in the catalyst [ 4,25,30-341, or if a chlorine-containing compound is introduced into the gas stream during the reaction [ 331. Of course, chlorine is well known to increase the selectivity of partial oxidation reactions, such as ethylene oxide from ethylene [35], formaldehyde from methane [36] and methanol from methane [ 37,381. Much of the currently published work on the promoting effects of halogens is based on experiments using very low pressures of methane or oxygen where high selectivity to C, products is more easily obtained. Also, the stability of some of the more selective catalysts is in doubt [ 341. The purpose of the present research was to investigate the role of gaseous chlorinated compounds as promoters of oxide and alkali doped oxide catalysts at higher reactant gas pressures, and to determine whether the activity could be regenerated by addition of such gaseous promoters. EXPERIMENTAL
Catalystpreparation
A variety of catalysts have been prepared using AnalaR reagents. They are listed in Table 1 together with their surface areas. The alkali doped oxide catalysts were prepared by the incipient wetness method using solutions of the appropriate alkali metal salts after which they were dried at 200’ C for 4 h and then calcined for 17 h in air at 750°C. The amount of alkali varied but was TABLE 1 Oxide catalysts
and alkali-promoted
Catalyst
Surface area/m”
SW&
3.7 1.5 7.2 1.4 n.d.* n.d.* 0.55 0.68
LiCl/Sm,O, MgO LiCl/MgO MnOl, LiCl/MnO, NaC1/MnOz LiCl/Li,CO, *Not determined.
oxide catalysts g-’
and their surface areas
71
typically 510% w/w. For convenience, the oxides are described by their formal oxidation states (e.g., MnO,). This does not imply that these specific compositions are stable under reaction conditions. Surface area determination The surface areas of the catalysts after calcination were determined by adsorption of nitrogen at - 196°C in a volumetric system. Catalyst testing The activity of the catalysts was determined using a quartz flow reactor (26 cm x 5 mm) mounted horizontally and heated by a programmable electric furnace. An amount of 100 mg of catalyst was used in each case and this was mounted in the reactor between quartz wool plugs. The gases used were methane (B.O.C. > 99.99%) and oxygen in nitrogen (B.O.C. special gases 5% oxygen/95% nitrogen. These were used without further purification. The flow of each gas was controlled by a Negretti flow controller set at 30 cm3 min-’ to give a final mixture having a methane: oxygen ratio of 2O:l. All testing was carried out at atmospheric pressure and at 750 ‘C. Chlorinated compounds (10 ~1) were injected through a heated port, mounted in the oxygen-nitrogen supply line. At fixed intervals, the reaction products were sampled using a gas sampling valve and analysed by a Perkin-Elmer Sigma FID gas chromatograph. The gas chromatograph was fitted with a Poropak N column operating at 65 aC. Analysis of carbon monoxide and carbon dioxide was made possible by incorporating a catalytic methanator into the gas chromatograph just before the flame ionization detector. This consisted of a Ni/ A1203 catalyst which converted carbon oxides into methane which could be detected by flame ionization in the normal way. Response factors were taken from literature values [ 391. The activity (A) of the catalyst is defined as percent conversion of methane into all products. The selectivity ( Si) to product i calculated as follows: Si= (mol CH, converted products) x 100%
to
product
i)/ (mol
CH,
converted
to
all
The space time yield (STY) is given by: STYi= [molar flow rate of CH4x (A/100) x (S,/lOO)]/g
catalyst
STY is expressed in mol gcat-l h-l. The experimental procedure was as follows. The catalyst sample was heated in the flowing reaction mixture to 750°C. This temperature was maintained for l-2 h until the activity and selectivity of the catalyst was essentially con-
72
stant. At this point the chlorinated compound was dosed over the catalyst. Analysis of the products began about 7 min after injection and sampling continued at regular intervals for about 1 h. No detailed attempts were made to analyse the products for chlorine-containing compounds. However, the evidence available indicates that the concentration of chlorine-containing compounds in the product mixture is very small.
RESULTS
The changes in selectivity and space time yield of ethylene and ethane after injection of the chlorinated compound, as a function of time, are shown in Fig. 1-8 for the various catalysts tested. The conversion of methane just prior to, and 7 min after, injection of dichloromethane is given in Table 2. The points at zero time in each of the figures represent the steady state selectivity and STY of products before injection of the chlorine-containing promoter. The dosing of the chlorinated compound began at time “zero”. Sm20, based catalysts
The points at zero time in Fig. 1 show that SmzO, was rather active at 750°C. The selectivity towards C2 products was 72.5% ( C&H4= 20.3% ) before addition of CH2C12. The C2 STY was about 0.07 mol/gcat/h. The catalyst was very stable during the first 2 h of heating. TABLE 2 Conversion of methane over various catalysts before and after injection of a pulse of CH,Cl, Catalyst
Conversion/% Before
After
11.7 5.2 7.6 3.8 5.0 3.7 5.5 3.8
6.0 3.8 9.3 3.0 13.4 15.1 11.6 9.7
Sm20, LiC1/Sm20, MgO LiCl/MgO MnO, LiCl/MnO* NaCl/MnO, MnO,” “Pulse of CHCl, used.
73
a selectlvlty I
/ %
t
0 0
60
60
time / min b STY / (mol/g-cat/hr)
r
I:
)c_____W_____*____________.__._____-___-.-----------*
0
60
60
time / mln
Fig. 1. (a)Selectivity of a Sm,O, catalyst as a function of time after injection of a pulse of CH,Cl,. (b)Space-time yields of a Sm,OB catalyst as a function of time after injection of a pulse of CH&l,. Key:(+)CO;(*)CO,;(O)C,H,;(x)C,H,.
Addition of CH2C12to Sm,O, had a marked effect (Fig. 1). The selectivity to COz, C,H, and C,H, was observed to decrease but selectivity to CO was found to increase. The STY curves show that the catalyst was much less active than before the addition of CH,Cl,. The effect of adding a single pulse of CHzCIB to Sm,O, is maintained at a more or less constant level for the duration of the experiment (1 h, i.e., long after the gaseous pulse has been flushed from the system). This indicates that the CH2C12 has induced a fundamental change in the nature of the catalyst, presumably by replacement of 02- or OH- groups at the surface by Cl- groups. By analogy with alumina-supported catalysts, the substitution of OH- by Clwould be expected to increase the surface acidity of the catalyst. Since it is known that the most selective catalysts for methane coupling are derived from oxides, an increase in acidity could account for the loss of selectivity to C2
74
products after the introduction of a pulse of CH,Cl,. The rate of formation of CO is hardly affected by the addition of CH#& whereas the rate of formation of all other products is significantly reduced. It is therefore possible that for this catalyst CO is produced on a different type of active site from that required to form Cz products or COz. LiCl/Sm20, For the LiC1/SmzOB catalyst the selectivity varied greatly during the first 2 h on stream. The initial C2 selectivity during the first 1 h on stream was 93.1% ( CzH, = 38.4% ), higher than that found for Sm,O, alone, However, the C, selectivity decreased sharply after this time. The C, selectivity after 2 h on stream was 58.1% (C,H,= 11.4%), which is even lower than for pure Sm,O,. The final activity and yield were also lower than was observed for the undoped SmsOs. A high initial activity and ethylene selectivity of LiCl/Sm,O, catalysts has already been reported by Otsuka et al. [34], but this is only a temporary effect. Thus, Otsuka et al. [32] have observed that LiCl-doped catalysts deactivate sharply during use and this is confirmed by our experiments. Addition of CH.J$ to the deactivated (i.e., “steady state”) LiC1/SmzO, catalyst (see Fig. 2 ) showed very interesting effects. The C, selectivity was immediately and dramatically increased to about 91.1% ( C&H4= 28.9% ) . Yield of both C2H4 and C2H6 were found to increase slightly, but then decreased gradually to their original values. Once steady state was attained the final activity was just below the initial activity before promotion by CH&12. The effect of CH.J& on LiCl/Sm,O, was completely different to its effect on Sm,O, (compare Figs. 1 and 2). In the case of LiCl/Sm,O,, the addition of CH,Cl, to the gas mixture promotes the formation of C2 products (especially C,H,) formation, and suppresses the formation of CO and COz. We thus have evidence that the addition of a chlorine-containing compound can regenerate some of the high selectivity originally found with a fresh alkali chloride-doped SmzO, catalyst. This may offer a means of maintaining these catalysts in the correct state to give high selectivity over long periods of operation. The poor stability of LiC1/Sm,Os catalysts has already been alluded to above. In the case of the LiC1/Sm,Os catalyst the major changes in selectivity due to the promoting effect on CH,Cl, are relatively short lived. Nevertheless, the addition of a chlorine containing compound significantly improves the selectivity for the formation of C, products over a Sm,O, catalyst providing this already contains an alkali metal. In the absence of the alkali metal the same treatment leads to enhanced combustion and a loss of selectivity to hydrocarbon products.
75
,ooselectlvlty
/ %
JzL
-I 60
16
0 time
/
min
STY / (mol/g-cat/hr) o.os’
I’ CA016
,Y.
**._
1 -
--._
-. ~_____________*________------
-* ----___
_.______)
time / mln Fig. 2. (a)Selectivity of a LiCl/Sm,O, catalyst as a function of time after injection of a pulse of CH,CI,. (b)Space-time yields of a LiC1/Sm,03 catalyst as a function of time after injection of a pulse of CH#&. Key: ( + ) CO; ( * ) COz; (0 ) C&H,; ( X ) C,H,.
MgO based catalysts
MgO
Fig. 3 (a) shows that our MgO catalyst was not as selective as Sm,O, for the formation of Cz products. The selectivity to C, was 62&O%, the major product being CzHs (ca. 50.7% ), the selectivity to C2H4 being very low at only 12.1%. Addition of CH,Cl, increased the yield of ethylene slightly, but, with a decrease in selectivity to ethane at the same time, the total C2 selectivity remained almost unchanged. The increases in the yields of C&H4and C,H, were very small and quickly returned to their original values. LiCljMgO
A LiCl/MgO catalyst was also investigated and it was found to have an initial selectivity to ethylene of 35.5%, which gradually decreased to a steady state
76
selectlvlty
/ %
mm
0 0
20 time
40 /
mln b
STY / (mol/g-cat/hr)
a04 r
0
20
40
time / mln Fig. 3. (a)Selectivity of a MgO catalyst as a function of time after injection of a pulse of CH,C&. (b)Space-time yields of a MgO catalyst as a function of time after injection of a pulse of CH,Cl,. Key: ( + ) CO; (*I CO,; (0 1 CY-L; ( X ) GH,.
value of 25.6%. At steady state, LiCl/MgO was less active, but more selective to Cz products (90.0% ) than unpromoted MgO. From Fig. 4, it is clear that the introduction of CHzClz to LiCl/MgO quickly increased the ethylene yield and selectivity, but that this then decreased again quite quickly. The magnitude of the increase was somewhat larger than for MgO alone. C2Hs selectivity decreased at the same time as the selectivity to C&H, increased, with the result that the total Cz selectivity was almost unchanged. The STY curves in Fig. 4 (b) show that the activity of LiCl/MgO decreased slightly on the addition of CH2C12,but that this change was soon reversed again. Addition of CHJX, to LiCl/MgO suppresses the formation of CO, but hardly affects the formation of CO. Overall these results show that the main effect of promoting a LiCl/MgO catalyst with CH,Cl, is to enhance the formation of
77
a selectlvtty
/ %
+I
0 0
40
20 the
*
* 60
min
/
b
STY / (rrol/g-cat/hr)
0
20
40 time
/
80
mln
Fig. 4. (a)Selectivity of a LiCl/MgO catalyst as a function of time after injection of a pulse of CH,Cl,. (b)Space-time yields of a LiCl/MgO catalyst as a function of time after injection of a pulse of CH,Cl,. Key: ( + ) CO; ( * ) CO,; ( q ) C&H,; ( x ) C,H,.
C&H, at the expense of C2H6. The CH2C12acts to promote the dehydrogenation reaction. MnO,-based catalysts MnO,, Fig. 5 (a) shows that MnOz generated virtually no ethylene at 750’ C and its selectivity to ethane was only 16.9%. It fact it was a stable total oxidation catalyst with a 82.6% selectivity to COz. However, as CH,Cl, was added to the catalyst (see Fig. 5 (a) ) there was a dramatic change in the product distribution. The total oxidation to COz was greatly inhibited (the selectivity to CO2 dropped to only 4.6% ) while the total selectivity to C2 products increased to 91.2% (C,H,=56.3%). The STY curves (see Fig. 5 (b) ) show clearly the in-
78
20
time / min b
0
20
40
time / min Fig. 5. (a)Selectivity of a MnO, catalyst as a function of time after injection of a pulse of CH,Cl,. (b)Space-time yields of a Mn02 catalyst as a function of time after injection of a pulse of CH,Cl,. Key: (+) CO; (*) CO,; (0) C,H,; (x) C,H,.
crease in the rate of formation of C, products and of CO also. The effect was sustained, although not at the initial high level of C, selectivity, for a comparatively long time (more than 1 h) after the injection of the CH2C12.Finally, it should be noted that the selectivity to CzH4 decreased more rapidly than the selectivity to CzH6. The contrast between MnOz and Sm20, or MgO catalysts is quite remarkable. Whereas with both Sm,03 and MgO the injection of CH2C12 had very little effect on the activity or product distribution, the effect of CHBClz on MnO, was very dramatic. Of all the catalysts tested only MnO, can be promoted by CH2C12in the absence of alkali. Two further significant effects of promoting MnO, with CH,Cl, are observed. First, the highest selectivity is not observed until about 14 min after injection of the pulse of CH2C12 (the pulse would take much less than 1 min to
79
pass through the bed of catalyst if there was no interaction). Second, the effect of the CH& is sustained for a much longer period of time than was the case with any of the Sm,O, or MgO catalysts. LiCl/Mn02 Promotion of MnOz by LiCl produced an active catalyst with a high initial C2 selectivity of 88.3% (C&H, = 48.6% ) . However, this activity was not maintained and the catalyst became deactivated during the first 2 h on stream with the C, selectivity falling to 54.5% (C,H,=9.6%). Addition of CH,Cl, to this deactivated LiC1/MnOp catalyst quickly restored the high initial Cz selectivity (see Fig. 6). It can be seen that the effect of the chlorine containing additive is similar for both MnO, and LiCl/MnO, in that it promoted higher yields of C,H, and C2H6. However, in contrast to MnOz, for the LiC1/MnOz catalyst
"0
40
20
60
time / mln
STY /
(lTKli/Q-CXt/hr)
time / min Fig. 6. (a)Selectivity of a LiCl/MnO, catalyst as a function of time after injection of a pulse of CH,Cl,. (b)Space-time yields of a LiCl/MnO, catalyst as a function of time after injection of a pulse of CH,Cl,. Key: ( + ) CO; ( * ) CO,; (0 ) C,H,; ( x )C,H,.
80
the selectivity to CO2 increased to beyond its original value within 1 h. Thus, in contrast to the Sm,O, catalysts the presence of lithium is not essential in order to develop the promoting effect of CH.&12. NaCl/MnO, NaCl/MnO:! was found to be a fairly stable catalyst compared with LiCl/ MnO,. There were no signs of deactivation within the first 2 h. The selectivity to C, products using NaCl/MnO, is about 87.8% ( CzH4= 22.9% ), much higher than in the case of LiC1/MnOz ( C2 selectivity = 54.5%, see above). Addition of CHJ& increased the selectivity to C&H, at the expense of both C2H6 and CO2 (see Fig. 7 (a) ); the total C2 selectivity increased to a maximum of 93.3%. From the STY curves (Fig. 7(b) ), it can be seen that the yields of ethylene and ethane were, in fact, both increased. The highest C, selectivity a ,ooselectlvlty
/ %
T 0
8 c
T
20
40
time / min b STY / (ml/g-cat/hr)
a
20
40 time / mln
Fig. 7. (a)Selectivity of a NaC1/Mn02 catalyst as a function of time after injection of a pulse of CH,Cl,. (b)Space-time yields of a NaCl/MnO, catalyst as a function of time after injection of a pulse of CH,Cl,. Key: ( + ) CO; ( + ) CO,; (0 ) C,H,; ( X ) C,H,.
81
detected was 93.3% (C2H4=44.9%) which is similar to the highest value for MnOB and LiCl/MnO* when promoted by CH,Cl,. In general it is observed that CH,Cl, greatly enhances the selectivity of MnOz-based catalysts for the formation of Cz products. Manganese oxide catalysts have been investigated before [ 1,20,21,33], but mainly using the technique of gas switching. Jones et al. [ 211 have shown that under those conditions alkali promotion of silica-supported MnO, is essential to avoid total oxidation. Our continuous flow experiments demonstrate that unsupported MnO,, promoted by LiCl or NaCl, has a much higher C, selectivity than MnOP on its own. However, we have also found that as an alternative to using alkali salts to increase C, selectivity, CH,Cl, is exceptionally effective in inhibiting the formation of carbon oxides over unsupported, undoped MnOz. Furthermore, CH&12 can also promote LiC1/MnOz and NaC1/MnOz to give an even higher selectivity and activity than before. Lithium-based catalysts. LiCl deposited on Li2C03 was tested to determine whether lithium alone can be promoted by CH,Cl, under our experimental conditions. It was observed that the catalyst was slightly promoted to give more C2H4, but that the selectivities soon returned to their original values. The results were similar to those obtained for LiCl/Sm,O, and LiCl/MgO. Promotion of manganese-based catalysts by CHC& The effect of promotion of CHCl, was also investigated and the trends observed were essentially identical to those found using CH&12. Representative data are given in Fig. 8 which shows the effect of promotion of MnOz by CHCl,. As expected there is a dramatic increase in the selectivity to Cz products with the formation of C2Hs in particular being sustained for quite a long time after the injection of the pulse of CHC13. The results presented for the various metal oxides and alkali chloride doped metal oxides show the different responses to promotion by CH&. There is a dramatic increase in selectivity and STY with respect to C, products for MnO,based catalysts. It is worth underlining the fact that the additional C, products are not simply produced from the CH,Cl, or its derivatives but really are formed from CH4. From Fig. 3 (b) it can be estimated that there were at least 3. 10e3 mol of C, products formed within 1 h. However, only about 1.6. lop4 mol of CH,C!l, were introduced, so the additional C2 products formed could not have derived from the CHzClz added. Furthermore, we have conducted an experiment using MnO, but this time without supplying methane to the catalyst while the pulse of
82
time / min b _STY
0
/ (mol/g-cat/hr)
20
40
60
time / min Fig. 8. (a) Selectivity of a MnO, catalyst as a function of time after injection of a pulse of CHCl,. (b)Space-time yields of a MnO, catalyst as a function of time after injection of a pulse of CHC13. Key: (+I CO; (*I CO,; (0) C&H.,(x) C2H6.
CH2C12was being introduced into the Nz-O2 stream. There was a considerable amount of CO and CO, which emerged from the reactor during the first few minutes after dosing with CH2C12.When there was no further sign of any carbon-containing products CH4 was again fed into the reactant stream. A high C, selectivity reappeared as expected. In order to confirm that the promotion effect is due to the chlorine rather than the hydrocarbon moiety of the chlorine containing promoter a further control experiment was carried out. A volume of 0.5 cm3 of chlorine was pulsed over a MnO, catalyst. This enhances C, selectivity and especially that of ethylene. Gaseous hydrogen chloride was also used to promote MnO, and this gave an even higher selectivity to ethylene than found with chlorine. A similar result has been briefly alluded to in a recent patent [ 331 which reported that hydrogen chloride can also promote NaC1/Mn02. These results are not consistent with the previously accepted view that C,
83
selectivity is enhanced under alkaline conditions [ 41,421. In our experiments an alkali-free MnOz catalyst can be made selective by treatment with hydrogen chloride, which in all probability will make the surface of the MnOz very acidic. It would appear that the dominant factor in determining the C, selectivity must be the presence of chlorine in the promoter. DISCUSSION
As discussed in the Introduction, recent work has indicated that chloridecontaining catalysts may be quite selective for the formation of C,H, and C_Hs by the direct oxidation of CH,. However, the most active catalyst, claimed by Otsuka [ 341 to be LiCl-doped Smz03, was also reported to lose its activity very rapidly during use. In the present work we have confirmed both these claims made by Otsuka high Cz selectivity of chlorine-containing Smz03 and rapid loss of activity. In addition, however, we have demonstrated that a substantial part of the C, selectivity can be recovered by passing a pulse of a chlorine-containing promoter (CH,Cl,) over the alkali-doped Sm,Os catalyst. We have also demonstrated that somewhat similar chlorine-promoting effects can be achieved with alkalidoped MgO catalysts, at least with regard to enhanced selectivity for the formation of C,H,. The influence of the chlorine-promotor is relatively short lived with these catalysts and presumably reflects the fact that chlorine will be lost continuously from the surface of these oxides under reaction conditions. Nevertheless, it may be possible to retain the high selectivity of these catalysts by the continual addition of very small quantities (ppm) of any of a wide variety of chlorine-containing compounds. However, in neither MgO nor Sm,03 is there any significant beneficial chloride effect in the absence of the alkali metal. This creates a potential problem since, for example, Sm,O,, is known to lose alkali halide under reaction conditions [ 341. Our results with MnOz catalysts are particularly important in this context since this case a very significant promotor effect of chlorine is observed in the absence of any alkali metal. Indeed, we have observed that pure MnO,, which is a very efficient total oxidation catalyst, can be converted into a very selective hydrocarbon synthesis catalyst by promotion by CH$&. Addition of alkali, and the choice of alkali, has an effect on the relative amounts of C2H4 and CzHGproduced over MnOz catalysts and on the way in which the quantities of these products change with time on stream after injecting the CH2C12.Table 3 summarizes some typical data. The results show that pure MnO, produces over 55% C&H, and almost 35% C&H615 min after the injection of CH,C$ as compared with no C,H, and only 16.9% C&H, initially. After 45 min the production of C&H4has declined to 21.2%
84
TABLE 3 Influence of CH,Cl, on the selectivity of MnO,-based catalysts for the formation of C2 products from CH, Catalyst
T/min*
C,H,**
MnO,
0 15 45 0 15 45 0 15 45
56.3 21.2 9.6 44.2 8.5 22.9 39.2 29.9
LiCl/MnO*
NaCI/MnO*
0
C&He***
C,”
16.9 34.9 66.2 44.9 44.3 40.0 64.9 53.5 59.9
91.2 87.4 54.5 88.5 48.5 87.8 92.7 89.8
16.9
*Time in minutes from injection of CH,Cl,. **Selectivity to ethylene. ***Selectivity to ethane. “Selectivity to C2 Products.
but the selectivity to C,H, has increased to 66.2%. Dehydrogenation has diminished in importance but selective coupling of CH, to C&H6has been retained. When LiCl is present, initially the catalyst produces 9.6% CzH4 and this increases to 44.2% 15 min after injection of CH&l, but then declines again to 8.5% after 45 min. The selectivity to C&H, shows hardly any change initially and then falls slightly (to 40.0%). In this catalyst the CH,Cl, only produces an increase in selectivity to C&H,. When NaCl is present, initially the catalyst has a high C2 selectivity (87.8% ) and injection of CH2C12results in the formation of much more C2H4, partly at the expense of C&H,. After a further 30 min the selectivity has returned to close to that of the original catalyst. In this case the role of the CH2C12promotor is mainly to develop the dehydrogenation capacity of the catalyst. It is notable that the alkali-free MnO, develops the highest selectivity for the formation of C&H, (56.3%) and that it then retains a high C, selectivity for 45 min. The enhanced C2 selectivity is eventually lost completely after about 6 h on stream. However, a further injection of a chlorine containing compound (CHC& in this case) recovers much of the C, selectivity (87.0% after 7 min). Clearly it is possible to promote MnO, with chlorine-containing compounds without requiring alkali metals to be present. Indeed, the best selectivity to C2H4 under our experimental conditions is obtained with the pure MnO, catalyst. It appears that chlorine inhibits the total oxidation and enhances the partial oxidation to C, products in MnO,-basedcatalysts and alkali-doped metal oxide catalysts. The different responses of the various catalysts leaves little doubt
85
that the overall reaction can not be purely homogeneous especially as there is no sign of C, formation when CHzClz is injected into an empty reactor. We believe that the promoting effect of chlorinated gaseous additives is very important. Our results show lithium-containing catalysts (LiC1/Sm,03, LiCl/ MgO and LiCl/Li,O,) can be promoted slightly by CH2C12,whereas alkali-free Sm,O, cannot. However, Mn02-based catalysts are substantially improved. We suggest that manganese and lithium ion promotors interact with CH,Cl, to form active chlorine species which catalyse the coupling reaction. MnO,based catalysts may interact more strongly with CH2C12than lithium-containing catalysts which may account for the higher activity and ethylene selectivity in Mn02-based catalysts. This effect may be related to the stability of active chlorine species on the surface of the catalyst. Although it is obvious that chlorine has the specific ability to improve the overall C2 selectivity and the CzH4 selectivity, it is still not clear what is the nature of the active catalytic species which are involved. It has been postulated that enhancement of ethylene selectivity can be ascribed to the presence of either active chlorine species [ 311, or modified oxygen species on the surface of the catalyst. It is also possible that the active chlorine species play a role in supplying chlorine radicals that are already known to catalyse the formation of ethylene from ethane [ 43,441. This is the explanation adopted by Otsuka [34] to account for the higher ethylene selectivity in LiCl-doped metal oxide catalysts. It is not yet clear whether this is a totally heterogeneous catalytic reaction or a heterogeneous-homogeneous reaction (an effect of chlorine radicals on the gas-phase reaction). Further studies are needed in order to understand the mechanism of the promotion by gaseous chlorinated compounds. In summary, chlorine-containing promotors produce two effects: (i) they enhance the dehydrogenation of ethane; (ii) they inhibit total oxidation, thus increasing the selectivity for the formation of C, products. These effects are observed even for pure, unpromoted manganese oxide catalysts. ACKNOWLEDGEMENTS
We are grateful to British Gas and SERC for supporting this research and to British Gas for providing financial support for G.D.S. We are pleased to acknowledge that S.C.T. is in receipt of an S.L. Pao Scholarship from Hong Kong.
REFERENCES 1 2 3
G.E. Keller and M.M. Bhasin, J. Catal., 73 (1982) 9. N.D. Parkyns, Symposium on Energy Production Processes, April 1988, Institution of Chemical Engineers, London, p. 35. CA. Jones, J.A. Sofranko and J.J. Leonard, Energy Fuels, 1 (198’7) 12.
86 4
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
W. Hinsen, W. Bytyn and M. Baerns, 8th Int. Congr. Catalysis, Berlin, 1984, DECHEMA, Vol. III, p. 581. T. Ito, Ji-X, Wang, C.-H, Lin and J.H. Lunsford, J. Am. Chem. Sot., 107 (1985) 5062. D.J. Driscoll and J.H. Lunsford, J. Phys. Chem., 89 (1985) 4415. I.T. Ali Emesh and Y. Amenomiya, J. Phys. Chem., 90 (1986) 4785. K.-I. Aika, T. Moriyama, N. Takasaki and E. Iwamatsu, J. Chem. Sot. Chem. Commun., (1986) 1210. K. Otsuka, K. Jinno and A. Morikawa, J. Catal., 100 (1986) 353. K. Otsuka, Q. Liu and A. Morikawa, J. Chem. Sot. Chem. Commun., (1986) 586. C.-H. Lin, K.D. Campbell, J.-X. Wang and J.H. Lunsford, J. Phys. Chem., 90 (1986) 534. H. Imai and T. Tagawa, J. Chem. Sot. Chem. Commun., (1986) 52. K. Otsuka and T. Komatsu, J. Chem. Sot. Chem. Commun., (1987) 388. H. Imai, T. Tagawa and N. Kamide, J. Catal., 106 (1987) 394. J.A. Labinger, K.C. Ott, S. Mehta, H.K. Rockstad and S. Zoumalan, J. Chem. Sot. Chem. Commun., (1987) 543. J.A. Labinger and K.C. Ott, J. Phys. Chem., 91 (1987) 2682. G.J. Hutchings, M.S. Scurrell and J.R. Woodhouse, J. Chem. Sot. Chem. Commun., (1987) 1862. S.J. Korf, J.A. Roos, N.A. de Bruijn, J.G. vam Ommen and J.R.H. Ross, J. Chem. Sot. Chem. Commun., (1987) 1433. E. Iwamatsu, T. Moriyama, N. Takaskai and K.-I. Aika, J. Chem. Sot. Chem. Commun., (1987) 19. J.A. Sofranko, J.J. Leonard and C.A. Jones, J. Catal., 103 (1987) 302. C.A. Jones, J.J. Leonardand J.A. Sofranko, J. Catal., 103 (1987) 311. G.J. Hutchings, M.S. Scurrell and J.R. Woodhouse, J. Chem. Sot. Chem. Commun., (1987) 1388. J.A. Roos, A.G. Bakker, H. Bosch, J.G. van Ommen and J.R.H. Ross, Catal. Today, 1 (1987) 133. S.J. Korf, J.A. Roos, N.A. de Bruijn, J.G. van Ommen and J.R.H. Ross, to be published in Catalysis Today. R. Burch, G.D. Squire, and SC. Tsang, Appl. Catal., 43 (1988) 105. I. Matsurura, Y. Utsumi, M. Nakai and T. Poi, Chem. Lett., (1986) 1981. K. Otsuka, Y. Shimizu and T. Komatsu, Chem. Lett., (1987) 1835. K. Otsuka, Q. Liu, M. Hatano and A. Moriawa, Chem. Lett., (1986) 467. T. Moriyama, N. Takasaki, E. Iwamatsu and K. Aika, Chem. Lett., (1986) 1165. P. Schneider and M. Bergfeld, European Workshop on Catalytic Methane Conversion, Bochum, May 1988. K. Otsuka, Q. Liu, M. Hatano and A. Morikawa, Chem. Lett., (1986)“B9 3. A.G. Anshits, N.P. Kirik, V.G. Roguleva, A.N. Shigapov and G.E. Selyutin, European Workshop on Catalytic Methane Conversion, Bochum, May 1988. Atlantic Richfield, U.S. Pat. 4, 634,800 (1987). K. Otsuka, Sekiyu Giakkaishi, 30 (1987) 385. J.M. Berty, Appl. Industrial Catal., 1 (1983) 207. C.F. Cullis, D.E. Keene and D.L. Trimm, J. Catal., 19 (1970) 378. M. Eusuf, A.M. Hasan, R. Khan and F.A. Chowdhury, Sci. Res., 10 (1968) 135. M. Eusuf, A.M. Hasan and R. Khan, Bangladesh J. Sci. Ind. Res., 10 (1975) 135. W.A. Dietz, J. Gas Chromatogr., (1967) 68. G.J. Hutchings, MS. Scurrel and J.R. Woodhouse, J. Chem. Sot., Chem. Commun., (1987) 1982. W. Bytyn and M. Baerns, Appl. Catal., 28 (1986) 199.
87 42 43 44
O.V. Buevskaya, A.I. Suleimanov, SM. Aliev and V.D. Sukolovski, React. Kinet. Catal. Lett. 33 (1987) 283. M. Weissman and S.W. Benson, Inter. J. Chem. Kinet., 16 (1984) 307. A. Granada, S.B. Karra and S.M. Senkan, Ind. Eng. Chem. Res., 26 (1987) 1901.
69
Printed in The Netherlands
Role of Chlorine in Improving Selectivity in the Oxidative Coupling of Methane to Ethylene R. BURCH*, G.D. SQUIRE and S.C. TSANG Catalysis Research Group, Chemistry Department, University of Reading, Whiteknights, P.O. Box 224, Reading RG6 2AD (U.K.) (Received 17 June 1988, revised manuscript received 2 September 1988)
ABSTRACT Samarium, magnesium and manganese oxide and alkali-promoted oxide catalysts have been prepared and tested for the oxidative coupling of methane. The results show that alkali-promoted oxides inhibit total oxidation and have a higher selectivity for the formation of C, products than the undoped metal oxides. These catalysts have been promoted by injecting pulses of gaseous chlorinated compounds (dichloromethane and chloroform) during the reaction. It has been found that these chlorinated compounds markedly increase the selectivity for the formation of C, products for all the MnOz-based catalysts and for lithium-doped MgO and SmLzOs catalysts. The effect is greatest in MnO*-based catalysts. When dichloromethane is added to a pure, unpromoted MnOz catalyst the selectivity for the formation of carbon dioxide decreases from 82.6% to 4.1% and the selectivity for the formation of C2H, increases from virtually zero to 56.3%. The highest C, selectivity observed after promotion of pure MnO, by dichloromethane is about 93%. Promotion of these pure oxide catalysts by gaseous chlorinated compounds provides an alternative to alkali promotion as a method of inhibiting total oxidation and of increasing ethylene production.
INTRODUCTION
The pioneering work of Keller and Bhasin [ 1 ] has stimulated interest in the oxidative coupling of methane. This is now recognised as an important first step in the synthesis of higher value hydrocarbons and has the potential of becoming an important industrial process [ 2 1. The fundamental requirement of the oxidative coupling reaction is to inhibit the thermodynamically favourable total oxidation to carbon oxides and enhance the selectivity to higher hydrocarbons principly ethane and ethylene. Keller and Bhasin [ 1]used a technique where methane and oxygen where fed over the catalyst alternately, so that it became correspondingly reduced or oxidised. This alternate feeding or switching of reactants was also adopted by Jones et al. [ 31 who claimed that it offered an increase in C, selectivity without using oxygen directly as the oxidant. However, many research workers use a different method of increasing selec-
0166-9834/89/$03.50
0 1989 Elsevier Science Publishers B.V.
70
tivity to C2 compounds. The reaction is carried out by co-feeding reactants over catalysts which are generally heavily doped with alkali or alkaline earth metal salts [ 4-291. Most work has been concentrated on the effect of different alkali metal dopants without considering the possible influence of the anions used to introduce the alkali metal. However, it has been reported recently that the highest selectivity to ethylene is achieved when chloride ions are present in the catalyst [ 4,25,30-341, or if a chlorine-containing compound is introduced into the gas stream during the reaction [ 331. Of course, chlorine is well known to increase the selectivity of partial oxidation reactions, such as ethylene oxide from ethylene [35], formaldehyde from methane [36] and methanol from methane [ 37,381. Much of the currently published work on the promoting effects of halogens is based on experiments using very low pressures of methane or oxygen where high selectivity to C, products is more easily obtained. Also, the stability of some of the more selective catalysts is in doubt [ 341. The purpose of the present research was to investigate the role of gaseous chlorinated compounds as promoters of oxide and alkali doped oxide catalysts at higher reactant gas pressures, and to determine whether the activity could be regenerated by addition of such gaseous promoters. EXPERIMENTAL
Catalystpreparation
A variety of catalysts have been prepared using AnalaR reagents. They are listed in Table 1 together with their surface areas. The alkali doped oxide catalysts were prepared by the incipient wetness method using solutions of the appropriate alkali metal salts after which they were dried at 200’ C for 4 h and then calcined for 17 h in air at 750°C. The amount of alkali varied but was TABLE 1 Oxide catalysts
and alkali-promoted
Catalyst
Surface area/m”
SW&
3.7 1.5 7.2 1.4 n.d.* n.d.* 0.55 0.68
LiCl/Sm,O, MgO LiCl/MgO MnOl, LiCl/MnO, NaC1/MnOz LiCl/Li,CO, *Not determined.
oxide catalysts g-’
and their surface areas
71
typically 510% w/w. For convenience, the oxides are described by their formal oxidation states (e.g., MnO,). This does not imply that these specific compositions are stable under reaction conditions. Surface area determination The surface areas of the catalysts after calcination were determined by adsorption of nitrogen at - 196°C in a volumetric system. Catalyst testing The activity of the catalysts was determined using a quartz flow reactor (26 cm x 5 mm) mounted horizontally and heated by a programmable electric furnace. An amount of 100 mg of catalyst was used in each case and this was mounted in the reactor between quartz wool plugs. The gases used were methane (B.O.C. > 99.99%) and oxygen in nitrogen (B.O.C. special gases 5% oxygen/95% nitrogen. These were used without further purification. The flow of each gas was controlled by a Negretti flow controller set at 30 cm3 min-’ to give a final mixture having a methane: oxygen ratio of 2O:l. All testing was carried out at atmospheric pressure and at 750 ‘C. Chlorinated compounds (10 ~1) were injected through a heated port, mounted in the oxygen-nitrogen supply line. At fixed intervals, the reaction products were sampled using a gas sampling valve and analysed by a Perkin-Elmer Sigma FID gas chromatograph. The gas chromatograph was fitted with a Poropak N column operating at 65 aC. Analysis of carbon monoxide and carbon dioxide was made possible by incorporating a catalytic methanator into the gas chromatograph just before the flame ionization detector. This consisted of a Ni/ A1203 catalyst which converted carbon oxides into methane which could be detected by flame ionization in the normal way. Response factors were taken from literature values [ 391. The activity (A) of the catalyst is defined as percent conversion of methane into all products. The selectivity ( Si) to product i calculated as follows: Si= (mol CH, converted products) x 100%
to
product
i)/ (mol
CH,
converted
to
all
The space time yield (STY) is given by: STYi= [molar flow rate of CH4x (A/100) x (S,/lOO)]/g
catalyst
STY is expressed in mol gcat-l h-l. The experimental procedure was as follows. The catalyst sample was heated in the flowing reaction mixture to 750°C. This temperature was maintained for l-2 h until the activity and selectivity of the catalyst was essentially con-
72
stant. At this point the chlorinated compound was dosed over the catalyst. Analysis of the products began about 7 min after injection and sampling continued at regular intervals for about 1 h. No detailed attempts were made to analyse the products for chlorine-containing compounds. However, the evidence available indicates that the concentration of chlorine-containing compounds in the product mixture is very small.
RESULTS
The changes in selectivity and space time yield of ethylene and ethane after injection of the chlorinated compound, as a function of time, are shown in Fig. 1-8 for the various catalysts tested. The conversion of methane just prior to, and 7 min after, injection of dichloromethane is given in Table 2. The points at zero time in each of the figures represent the steady state selectivity and STY of products before injection of the chlorine-containing promoter. The dosing of the chlorinated compound began at time “zero”. Sm20, based catalysts
The points at zero time in Fig. 1 show that SmzO, was rather active at 750°C. The selectivity towards C2 products was 72.5% ( C&H4= 20.3% ) before addition of CH2C12. The C2 STY was about 0.07 mol/gcat/h. The catalyst was very stable during the first 2 h of heating. TABLE 2 Conversion of methane over various catalysts before and after injection of a pulse of CH,Cl, Catalyst
Conversion/% Before
After
11.7 5.2 7.6 3.8 5.0 3.7 5.5 3.8
6.0 3.8 9.3 3.0 13.4 15.1 11.6 9.7
Sm20, LiC1/Sm20, MgO LiCl/MgO MnO, LiCl/MnO* NaCl/MnO, MnO,” “Pulse of CHCl, used.
73
a selectlvlty I
/ %
t
0 0
60
60
time / min b STY / (mol/g-cat/hr)
r
I:
)c_____W_____*____________.__._____-___-.-----------*
0
60
60
time / mln
Fig. 1. (a)Selectivity of a Sm,O, catalyst as a function of time after injection of a pulse of CH,Cl,. (b)Space-time yields of a Sm,OB catalyst as a function of time after injection of a pulse of CH&l,. Key:(+)CO;(*)CO,;(O)C,H,;(x)C,H,.
Addition of CH2C12to Sm,O, had a marked effect (Fig. 1). The selectivity to COz, C,H, and C,H, was observed to decrease but selectivity to CO was found to increase. The STY curves show that the catalyst was much less active than before the addition of CH,Cl,. The effect of adding a single pulse of CHzCIB to Sm,O, is maintained at a more or less constant level for the duration of the experiment (1 h, i.e., long after the gaseous pulse has been flushed from the system). This indicates that the CH2C12 has induced a fundamental change in the nature of the catalyst, presumably by replacement of 02- or OH- groups at the surface by Cl- groups. By analogy with alumina-supported catalysts, the substitution of OH- by Clwould be expected to increase the surface acidity of the catalyst. Since it is known that the most selective catalysts for methane coupling are derived from oxides, an increase in acidity could account for the loss of selectivity to C2
74
products after the introduction of a pulse of CH,Cl,. The rate of formation of CO is hardly affected by the addition of CH#& whereas the rate of formation of all other products is significantly reduced. It is therefore possible that for this catalyst CO is produced on a different type of active site from that required to form Cz products or COz. LiCl/Sm20, For the LiC1/SmzOB catalyst the selectivity varied greatly during the first 2 h on stream. The initial C2 selectivity during the first 1 h on stream was 93.1% ( CzH, = 38.4% ), higher than that found for Sm,O, alone, However, the C, selectivity decreased sharply after this time. The C, selectivity after 2 h on stream was 58.1% (C,H,= 11.4%), which is even lower than for pure Sm,O,. The final activity and yield were also lower than was observed for the undoped SmsOs. A high initial activity and ethylene selectivity of LiCl/Sm,O, catalysts has already been reported by Otsuka et al. [34], but this is only a temporary effect. Thus, Otsuka et al. [32] have observed that LiCl-doped catalysts deactivate sharply during use and this is confirmed by our experiments. Addition of CH.J$ to the deactivated (i.e., “steady state”) LiC1/SmzO, catalyst (see Fig. 2 ) showed very interesting effects. The C, selectivity was immediately and dramatically increased to about 91.1% ( C&H4= 28.9% ) . Yield of both C2H4 and C2H6 were found to increase slightly, but then decreased gradually to their original values. Once steady state was attained the final activity was just below the initial activity before promotion by CH&12. The effect of CH.J& on LiCl/Sm,O, was completely different to its effect on Sm,O, (compare Figs. 1 and 2). In the case of LiCl/Sm,O,, the addition of CH,Cl, to the gas mixture promotes the formation of C2 products (especially C,H,) formation, and suppresses the formation of CO and COz. We thus have evidence that the addition of a chlorine-containing compound can regenerate some of the high selectivity originally found with a fresh alkali chloride-doped SmzO, catalyst. This may offer a means of maintaining these catalysts in the correct state to give high selectivity over long periods of operation. The poor stability of LiC1/Sm,Os catalysts has already been alluded to above. In the case of the LiC1/Sm,Os catalyst the major changes in selectivity due to the promoting effect on CH,Cl, are relatively short lived. Nevertheless, the addition of a chlorine containing compound significantly improves the selectivity for the formation of C, products over a Sm,O, catalyst providing this already contains an alkali metal. In the absence of the alkali metal the same treatment leads to enhanced combustion and a loss of selectivity to hydrocarbon products.
75
,ooselectlvlty
/ %
JzL
-I 60
16
0 time
/
min
STY / (mol/g-cat/hr) o.os’
I’ CA016
,Y.
**._
1 -
--._
-. ~_____________*________------
-* ----___
_.______)
time / mln Fig. 2. (a)Selectivity of a LiCl/Sm,O, catalyst as a function of time after injection of a pulse of CH,CI,. (b)Space-time yields of a LiC1/Sm,03 catalyst as a function of time after injection of a pulse of CH#&. Key: ( + ) CO; ( * ) COz; (0 ) C&H,; ( X ) C,H,.
MgO based catalysts
MgO
Fig. 3 (a) shows that our MgO catalyst was not as selective as Sm,O, for the formation of Cz products. The selectivity to C, was 62&O%, the major product being CzHs (ca. 50.7% ), the selectivity to C2H4 being very low at only 12.1%. Addition of CH,Cl, increased the yield of ethylene slightly, but, with a decrease in selectivity to ethane at the same time, the total C2 selectivity remained almost unchanged. The increases in the yields of C&H4and C,H, were very small and quickly returned to their original values. LiCljMgO
A LiCl/MgO catalyst was also investigated and it was found to have an initial selectivity to ethylene of 35.5%, which gradually decreased to a steady state
76
selectlvlty
/ %
mm
0 0
20 time
40 /
mln b
STY / (mol/g-cat/hr)
a04 r
0
20
40
time / mln Fig. 3. (a)Selectivity of a MgO catalyst as a function of time after injection of a pulse of CH,C&. (b)Space-time yields of a MgO catalyst as a function of time after injection of a pulse of CH,Cl,. Key: ( + ) CO; (*I CO,; (0 1 CY-L; ( X ) GH,.
value of 25.6%. At steady state, LiCl/MgO was less active, but more selective to Cz products (90.0% ) than unpromoted MgO. From Fig. 4, it is clear that the introduction of CHzClz to LiCl/MgO quickly increased the ethylene yield and selectivity, but that this then decreased again quite quickly. The magnitude of the increase was somewhat larger than for MgO alone. C2Hs selectivity decreased at the same time as the selectivity to C&H, increased, with the result that the total Cz selectivity was almost unchanged. The STY curves in Fig. 4 (b) show that the activity of LiCl/MgO decreased slightly on the addition of CH2C12,but that this change was soon reversed again. Addition of CHJX, to LiCl/MgO suppresses the formation of CO, but hardly affects the formation of CO. Overall these results show that the main effect of promoting a LiCl/MgO catalyst with CH,Cl, is to enhance the formation of
77
a selectlvtty
/ %
+I
0 0
40
20 the
*
* 60
min
/
b
STY / (rrol/g-cat/hr)
0
20
40 time
/
80
mln
Fig. 4. (a)Selectivity of a LiCl/MgO catalyst as a function of time after injection of a pulse of CH,Cl,. (b)Space-time yields of a LiCl/MgO catalyst as a function of time after injection of a pulse of CH,Cl,. Key: ( + ) CO; ( * ) CO,; ( q ) C&H,; ( x ) C,H,.
C&H, at the expense of C2H6. The CH2C12acts to promote the dehydrogenation reaction. MnO,-based catalysts MnO,, Fig. 5 (a) shows that MnOz generated virtually no ethylene at 750’ C and its selectivity to ethane was only 16.9%. It fact it was a stable total oxidation catalyst with a 82.6% selectivity to COz. However, as CH,Cl, was added to the catalyst (see Fig. 5 (a) ) there was a dramatic change in the product distribution. The total oxidation to COz was greatly inhibited (the selectivity to CO2 dropped to only 4.6% ) while the total selectivity to C2 products increased to 91.2% (C,H,=56.3%). The STY curves (see Fig. 5 (b) ) show clearly the in-
78
20
time / min b
0
20
40
time / min Fig. 5. (a)Selectivity of a MnO, catalyst as a function of time after injection of a pulse of CH,Cl,. (b)Space-time yields of a Mn02 catalyst as a function of time after injection of a pulse of CH,Cl,. Key: (+) CO; (*) CO,; (0) C,H,; (x) C,H,.
crease in the rate of formation of C, products and of CO also. The effect was sustained, although not at the initial high level of C, selectivity, for a comparatively long time (more than 1 h) after the injection of the CH2C12.Finally, it should be noted that the selectivity to CzH4 decreased more rapidly than the selectivity to CzH6. The contrast between MnOz and Sm20, or MgO catalysts is quite remarkable. Whereas with both Sm,03 and MgO the injection of CH2C12 had very little effect on the activity or product distribution, the effect of CHBClz on MnO, was very dramatic. Of all the catalysts tested only MnO, can be promoted by CH2C12in the absence of alkali. Two further significant effects of promoting MnO, with CH,Cl, are observed. First, the highest selectivity is not observed until about 14 min after injection of the pulse of CH2C12 (the pulse would take much less than 1 min to
79
pass through the bed of catalyst if there was no interaction). Second, the effect of the CH& is sustained for a much longer period of time than was the case with any of the Sm,O, or MgO catalysts. LiCl/Mn02 Promotion of MnOz by LiCl produced an active catalyst with a high initial C2 selectivity of 88.3% (C&H, = 48.6% ) . However, this activity was not maintained and the catalyst became deactivated during the first 2 h on stream with the C, selectivity falling to 54.5% (C,H,=9.6%). Addition of CH,Cl, to this deactivated LiC1/MnOp catalyst quickly restored the high initial Cz selectivity (see Fig. 6). It can be seen that the effect of the chlorine containing additive is similar for both MnO, and LiCl/MnO, in that it promoted higher yields of C,H, and C2H6. However, in contrast to MnOz, for the LiC1/MnOz catalyst
"0
40
20
60
time / mln
STY /
(lTKli/Q-CXt/hr)
time / min Fig. 6. (a)Selectivity of a LiCl/MnO, catalyst as a function of time after injection of a pulse of CH,Cl,. (b)Space-time yields of a LiCl/MnO, catalyst as a function of time after injection of a pulse of CH,Cl,. Key: ( + ) CO; ( * ) CO,; (0 ) C,H,; ( x )C,H,.
80
the selectivity to CO2 increased to beyond its original value within 1 h. Thus, in contrast to the Sm,O, catalysts the presence of lithium is not essential in order to develop the promoting effect of CH.&12. NaCl/MnO, NaCl/MnO:! was found to be a fairly stable catalyst compared with LiCl/ MnO,. There were no signs of deactivation within the first 2 h. The selectivity to C, products using NaCl/MnO, is about 87.8% ( CzH4= 22.9% ), much higher than in the case of LiC1/MnOz ( C2 selectivity = 54.5%, see above). Addition of CHJ& increased the selectivity to C&H, at the expense of both C2H6 and CO2 (see Fig. 7 (a) ); the total C2 selectivity increased to a maximum of 93.3%. From the STY curves (Fig. 7(b) ), it can be seen that the yields of ethylene and ethane were, in fact, both increased. The highest C, selectivity a ,ooselectlvlty
/ %
T 0
8 c
T
20
40
time / min b STY / (ml/g-cat/hr)
a
20
40 time / mln
Fig. 7. (a)Selectivity of a NaC1/Mn02 catalyst as a function of time after injection of a pulse of CH,Cl,. (b)Space-time yields of a NaCl/MnO, catalyst as a function of time after injection of a pulse of CH,Cl,. Key: ( + ) CO; ( + ) CO,; (0 ) C,H,; ( X ) C,H,.
81
detected was 93.3% (C2H4=44.9%) which is similar to the highest value for MnOB and LiCl/MnO* when promoted by CH,Cl,. In general it is observed that CH,Cl, greatly enhances the selectivity of MnOz-based catalysts for the formation of Cz products. Manganese oxide catalysts have been investigated before [ 1,20,21,33], but mainly using the technique of gas switching. Jones et al. [ 211 have shown that under those conditions alkali promotion of silica-supported MnO, is essential to avoid total oxidation. Our continuous flow experiments demonstrate that unsupported MnO,, promoted by LiCl or NaCl, has a much higher C, selectivity than MnOP on its own. However, we have also found that as an alternative to using alkali salts to increase C, selectivity, CH,Cl, is exceptionally effective in inhibiting the formation of carbon oxides over unsupported, undoped MnOz. Furthermore, CH&12 can also promote LiC1/MnOz and NaC1/MnOz to give an even higher selectivity and activity than before. Lithium-based catalysts. LiCl deposited on Li2C03 was tested to determine whether lithium alone can be promoted by CH,Cl, under our experimental conditions. It was observed that the catalyst was slightly promoted to give more C2H4, but that the selectivities soon returned to their original values. The results were similar to those obtained for LiCl/Sm,O, and LiCl/MgO. Promotion of manganese-based catalysts by CHC& The effect of promotion of CHCl, was also investigated and the trends observed were essentially identical to those found using CH&12. Representative data are given in Fig. 8 which shows the effect of promotion of MnOz by CHCl,. As expected there is a dramatic increase in the selectivity to Cz products with the formation of C2Hs in particular being sustained for quite a long time after the injection of the pulse of CHC13. The results presented for the various metal oxides and alkali chloride doped metal oxides show the different responses to promotion by CH&. There is a dramatic increase in selectivity and STY with respect to C, products for MnO,based catalysts. It is worth underlining the fact that the additional C, products are not simply produced from the CH,Cl, or its derivatives but really are formed from CH4. From Fig. 3 (b) it can be estimated that there were at least 3. 10e3 mol of C, products formed within 1 h. However, only about 1.6. lop4 mol of CH,C!l, were introduced, so the additional C2 products formed could not have derived from the CHzClz added. Furthermore, we have conducted an experiment using MnO, but this time without supplying methane to the catalyst while the pulse of
82
time / min b _STY
0
/ (mol/g-cat/hr)
20
40
60
time / min Fig. 8. (a) Selectivity of a MnO, catalyst as a function of time after injection of a pulse of CHCl,. (b)Space-time yields of a MnO, catalyst as a function of time after injection of a pulse of CHC13. Key: (+I CO; (*I CO,; (0) C&H.,(x) C2H6.
CH2C12was being introduced into the Nz-O2 stream. There was a considerable amount of CO and CO, which emerged from the reactor during the first few minutes after dosing with CH2C12.When there was no further sign of any carbon-containing products CH4 was again fed into the reactant stream. A high C, selectivity reappeared as expected. In order to confirm that the promotion effect is due to the chlorine rather than the hydrocarbon moiety of the chlorine containing promoter a further control experiment was carried out. A volume of 0.5 cm3 of chlorine was pulsed over a MnO, catalyst. This enhances C, selectivity and especially that of ethylene. Gaseous hydrogen chloride was also used to promote MnO, and this gave an even higher selectivity to ethylene than found with chlorine. A similar result has been briefly alluded to in a recent patent [ 331 which reported that hydrogen chloride can also promote NaC1/Mn02. These results are not consistent with the previously accepted view that C,
83
selectivity is enhanced under alkaline conditions [ 41,421. In our experiments an alkali-free MnOz catalyst can be made selective by treatment with hydrogen chloride, which in all probability will make the surface of the MnOz very acidic. It would appear that the dominant factor in determining the C, selectivity must be the presence of chlorine in the promoter. DISCUSSION
As discussed in the Introduction, recent work has indicated that chloridecontaining catalysts may be quite selective for the formation of C,H, and C_Hs by the direct oxidation of CH,. However, the most active catalyst, claimed by Otsuka [ 341 to be LiCl-doped Smz03, was also reported to lose its activity very rapidly during use. In the present work we have confirmed both these claims made by Otsuka high Cz selectivity of chlorine-containing Smz03 and rapid loss of activity. In addition, however, we have demonstrated that a substantial part of the C, selectivity can be recovered by passing a pulse of a chlorine-containing promoter (CH,Cl,) over the alkali-doped Sm,Os catalyst. We have also demonstrated that somewhat similar chlorine-promoting effects can be achieved with alkalidoped MgO catalysts, at least with regard to enhanced selectivity for the formation of C,H,. The influence of the chlorine-promotor is relatively short lived with these catalysts and presumably reflects the fact that chlorine will be lost continuously from the surface of these oxides under reaction conditions. Nevertheless, it may be possible to retain the high selectivity of these catalysts by the continual addition of very small quantities (ppm) of any of a wide variety of chlorine-containing compounds. However, in neither MgO nor Sm,03 is there any significant beneficial chloride effect in the absence of the alkali metal. This creates a potential problem since, for example, Sm,O,, is known to lose alkali halide under reaction conditions [ 341. Our results with MnOz catalysts are particularly important in this context since this case a very significant promotor effect of chlorine is observed in the absence of any alkali metal. Indeed, we have observed that pure MnO,, which is a very efficient total oxidation catalyst, can be converted into a very selective hydrocarbon synthesis catalyst by promotion by CH$&. Addition of alkali, and the choice of alkali, has an effect on the relative amounts of C2H4 and CzHGproduced over MnOz catalysts and on the way in which the quantities of these products change with time on stream after injecting the CH2C12.Table 3 summarizes some typical data. The results show that pure MnO, produces over 55% C&H, and almost 35% C&H615 min after the injection of CH,C$ as compared with no C,H, and only 16.9% C&H, initially. After 45 min the production of C&H4has declined to 21.2%
84
TABLE 3 Influence of CH,Cl, on the selectivity of MnO,-based catalysts for the formation of C2 products from CH, Catalyst
T/min*
C,H,**
MnO,
0 15 45 0 15 45 0 15 45
56.3 21.2 9.6 44.2 8.5 22.9 39.2 29.9
LiCl/MnO*
NaCI/MnO*
0
C&He***
C,”
16.9 34.9 66.2 44.9 44.3 40.0 64.9 53.5 59.9
91.2 87.4 54.5 88.5 48.5 87.8 92.7 89.8
16.9
*Time in minutes from injection of CH,Cl,. **Selectivity to ethylene. ***Selectivity to ethane. “Selectivity to C2 Products.
but the selectivity to C,H, has increased to 66.2%. Dehydrogenation has diminished in importance but selective coupling of CH, to C&H6has been retained. When LiCl is present, initially the catalyst produces 9.6% CzH4 and this increases to 44.2% 15 min after injection of CH&l, but then declines again to 8.5% after 45 min. The selectivity to C&H, shows hardly any change initially and then falls slightly (to 40.0%). In this catalyst the CH,Cl, only produces an increase in selectivity to C&H,. When NaCl is present, initially the catalyst has a high C2 selectivity (87.8% ) and injection of CH2C12results in the formation of much more C2H4, partly at the expense of C&H,. After a further 30 min the selectivity has returned to close to that of the original catalyst. In this case the role of the CH2C12promotor is mainly to develop the dehydrogenation capacity of the catalyst. It is notable that the alkali-free MnO, develops the highest selectivity for the formation of C&H, (56.3%) and that it then retains a high C, selectivity for 45 min. The enhanced C2 selectivity is eventually lost completely after about 6 h on stream. However, a further injection of a chlorine containing compound (CHC& in this case) recovers much of the C, selectivity (87.0% after 7 min). Clearly it is possible to promote MnO, with chlorine-containing compounds without requiring alkali metals to be present. Indeed, the best selectivity to C2H4 under our experimental conditions is obtained with the pure MnO, catalyst. It appears that chlorine inhibits the total oxidation and enhances the partial oxidation to C, products in MnO,-basedcatalysts and alkali-doped metal oxide catalysts. The different responses of the various catalysts leaves little doubt
85
that the overall reaction can not be purely homogeneous especially as there is no sign of C, formation when CHzClz is injected into an empty reactor. We believe that the promoting effect of chlorinated gaseous additives is very important. Our results show lithium-containing catalysts (LiC1/Sm,03, LiCl/ MgO and LiCl/Li,O,) can be promoted slightly by CH2C12,whereas alkali-free Sm,O, cannot. However, Mn02-based catalysts are substantially improved. We suggest that manganese and lithium ion promotors interact with CH,Cl, to form active chlorine species which catalyse the coupling reaction. MnO,based catalysts may interact more strongly with CH2C12than lithium-containing catalysts which may account for the higher activity and ethylene selectivity in Mn02-based catalysts. This effect may be related to the stability of active chlorine species on the surface of the catalyst. Although it is obvious that chlorine has the specific ability to improve the overall C2 selectivity and the CzH4 selectivity, it is still not clear what is the nature of the active catalytic species which are involved. It has been postulated that enhancement of ethylene selectivity can be ascribed to the presence of either active chlorine species [ 311, or modified oxygen species on the surface of the catalyst. It is also possible that the active chlorine species play a role in supplying chlorine radicals that are already known to catalyse the formation of ethylene from ethane [ 43,441. This is the explanation adopted by Otsuka [34] to account for the higher ethylene selectivity in LiCl-doped metal oxide catalysts. It is not yet clear whether this is a totally heterogeneous catalytic reaction or a heterogeneous-homogeneous reaction (an effect of chlorine radicals on the gas-phase reaction). Further studies are needed in order to understand the mechanism of the promotion by gaseous chlorinated compounds. In summary, chlorine-containing promotors produce two effects: (i) they enhance the dehydrogenation of ethane; (ii) they inhibit total oxidation, thus increasing the selectivity for the formation of C, products. These effects are observed even for pure, unpromoted manganese oxide catalysts. ACKNOWLEDGEMENTS
We are grateful to British Gas and SERC for supporting this research and to British Gas for providing financial support for G.D.S. We are pleased to acknowledge that S.C.T. is in receipt of an S.L. Pao Scholarship from Hong Kong.
REFERENCES 1 2 3
G.E. Keller and M.M. Bhasin, J. Catal., 73 (1982) 9. N.D. Parkyns, Symposium on Energy Production Processes, April 1988, Institution of Chemical Engineers, London, p. 35. CA. Jones, J.A. Sofranko and J.J. Leonard, Energy Fuels, 1 (198’7) 12.
86 4
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
W. Hinsen, W. Bytyn and M. Baerns, 8th Int. Congr. Catalysis, Berlin, 1984, DECHEMA, Vol. III, p. 581. T. Ito, Ji-X, Wang, C.-H, Lin and J.H. Lunsford, J. Am. Chem. Sot., 107 (1985) 5062. D.J. Driscoll and J.H. Lunsford, J. Phys. Chem., 89 (1985) 4415. I.T. Ali Emesh and Y. Amenomiya, J. Phys. Chem., 90 (1986) 4785. K.-I. Aika, T. Moriyama, N. Takasaki and E. Iwamatsu, J. Chem. Sot. Chem. Commun., (1986) 1210. K. Otsuka, K. Jinno and A. Morikawa, J. Catal., 100 (1986) 353. K. Otsuka, Q. Liu and A. Morikawa, J. Chem. Sot. Chem. Commun., (1986) 586. C.-H. Lin, K.D. Campbell, J.-X. Wang and J.H. Lunsford, J. Phys. Chem., 90 (1986) 534. H. Imai and T. Tagawa, J. Chem. Sot. Chem. Commun., (1986) 52. K. Otsuka and T. Komatsu, J. Chem. Sot. Chem. Commun., (1987) 388. H. Imai, T. Tagawa and N. Kamide, J. Catal., 106 (1987) 394. J.A. Labinger, K.C. Ott, S. Mehta, H.K. Rockstad and S. Zoumalan, J. Chem. Sot. Chem. Commun., (1987) 543. J.A. Labinger and K.C. Ott, J. Phys. Chem., 91 (1987) 2682. G.J. Hutchings, M.S. Scurrell and J.R. Woodhouse, J. Chem. Sot. Chem. Commun., (1987) 1862. S.J. Korf, J.A. Roos, N.A. de Bruijn, J.G. vam Ommen and J.R.H. Ross, J. Chem. Sot. Chem. Commun., (1987) 1433. E. Iwamatsu, T. Moriyama, N. Takaskai and K.-I. Aika, J. Chem. Sot. Chem. Commun., (1987) 19. J.A. Sofranko, J.J. Leonard and C.A. Jones, J. Catal., 103 (1987) 302. C.A. Jones, J.J. Leonardand J.A. Sofranko, J. Catal., 103 (1987) 311. G.J. Hutchings, M.S. Scurrell and J.R. Woodhouse, J. Chem. Sot. Chem. Commun., (1987) 1388. J.A. Roos, A.G. Bakker, H. Bosch, J.G. van Ommen and J.R.H. Ross, Catal. Today, 1 (1987) 133. S.J. Korf, J.A. Roos, N.A. de Bruijn, J.G. van Ommen and J.R.H. Ross, to be published in Catalysis Today. R. Burch, G.D. Squire, and SC. Tsang, Appl. Catal., 43 (1988) 105. I. Matsurura, Y. Utsumi, M. Nakai and T. Poi, Chem. Lett., (1986) 1981. K. Otsuka, Y. Shimizu and T. Komatsu, Chem. Lett., (1987) 1835. K. Otsuka, Q. Liu, M. Hatano and A. Moriawa, Chem. Lett., (1986) 467. T. Moriyama, N. Takasaki, E. Iwamatsu and K. Aika, Chem. Lett., (1986) 1165. P. Schneider and M. Bergfeld, European Workshop on Catalytic Methane Conversion, Bochum, May 1988. K. Otsuka, Q. Liu, M. Hatano and A. Morikawa, Chem. Lett., (1986)“B9 3. A.G. Anshits, N.P. Kirik, V.G. Roguleva, A.N. Shigapov and G.E. Selyutin, European Workshop on Catalytic Methane Conversion, Bochum, May 1988. Atlantic Richfield, U.S. Pat. 4, 634,800 (1987). K. Otsuka, Sekiyu Giakkaishi, 30 (1987) 385. J.M. Berty, Appl. Industrial Catal., 1 (1983) 207. C.F. Cullis, D.E. Keene and D.L. Trimm, J. Catal., 19 (1970) 378. M. Eusuf, A.M. Hasan, R. Khan and F.A. Chowdhury, Sci. Res., 10 (1968) 135. M. Eusuf, A.M. Hasan and R. Khan, Bangladesh J. Sci. Ind. Res., 10 (1975) 135. W.A. Dietz, J. Gas Chromatogr., (1967) 68. G.J. Hutchings, MS. Scurrel and J.R. Woodhouse, J. Chem. Sot., Chem. Commun., (1987) 1982. W. Bytyn and M. Baerns, Appl. Catal., 28 (1986) 199.
87 42 43 44
O.V. Buevskaya, A.I. Suleimanov, SM. Aliev and V.D. Sukolovski, React. Kinet. Catal. Lett. 33 (1987) 283. M. Weissman and S.W. Benson, Inter. J. Chem. Kinet., 16 (1984) 307. A. Granada, S.B. Karra and S.M. Senkan, Ind. Eng. Chem. Res., 26 (1987) 1901.