132 Chem. Eng. Technol. 18 (1995) 132-138

An Inverse Correlation of the Enhancement Effect of Tetrachloromethane as a Feedstream Additive in the Oxidative Coupling of Methane on Silica-Supported Alkaline Earth and Alkali-Alkaline Earth Catalysts with the Polarizing Ability of the Alkaline Earth Cations

Shamsuddin Ahmed and John B. Moffat*

Dedicated to Professor Dr. Manfred Buerns on the occasion of his 60th birthday

Results are reported for the oxidative coupling of methane on silica-supported alkaline earths prepared from either their acetates or chlorides with and without alkali metals as dopants and in the absence or presence of carbon tetrachloride (TCM). The addition of small quantities of TCM to the methane feedstream produces increases in the conversion of methane and the selectivities to C2 hydrocarbons which correlate with the increase in cation size and thus are inversely related to the polarizing abilities of the cations.

1 Introduction

For more than a decade the conversion of natural gas, i.e. methane, has commanded the attention of workers in catal- ysis [ 1 - 81. There was (and still is) general agreement that natural gas is an underutilized resource worldwide. Much of the current consumption of natural gas occurs in the genera- tion of heat through its complete, or nearly complete, com- bustion or in the production of carbon monoxide and hy- drogen. While methane may ultimately be converted to syn- thetic crude, via the Fischer-Tropsch process, methanol or gasoline, the latter by means of the MTG process, or ammo- nia, nevertheless all of these processes involve two or more steps in the formation of the desired material. Thus it is not unnatural that considerable interest developed in the possi- bility of converting methane to liquid fuels or to precursors of value added chemicals.

As is well known, the dimerization of methane to ethane is thermodynamically disallowed. While the addition of ox- ygen produces the required spontaneity it unfortunately also introduces the possibility of competing deep oxidation processes. Thus, not unexpectedly, catalysis research has focused on the partial oxidation to formaldehyde and meth- anol and oxidative coupling to ethane and ethylene. Al- though the chemistry of these processes has proven to be less cooperative than originally anticipated, and conse- quently some diminution in interest in the catalytic aspects has occurred, nevertheless there is some hope that engineer- ing developments may facilitate the large scale implementa- tion of one or more of the direct processes [S]. However, from a purely fundamental viewpoint the knowledge gained

* S. Ahmed and Prof. J.B. Moffat (to whom correspondence should be addressed), Department of Chemistry and the Guelph-Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1.

from studies of these direct processes has enhanced our knowledge of oxidation processes and catalysis in general and will continue to do so.

The activation of methane, and ultimately the scission of the C - H bond is a necessary, but not a sufficient, condi- tion for either the partial oxidation or the oxidative cou- pling processes. That suitable catalysts activate methane ap- pears to be widely accepted, although disadvantageous pro- cesses may also occur heterogeneously [ 1 - 81. While a vari- ety of active sites has been proposed no consensus has emerged from the data presently available [l - 81.

The properties required for an active and selective catalyst are of obvious importance in studies of the conversion of methane. Baerns and coworkers have suggested that the basicity of the catalytic solids is an important factor in the process and have illustrated this with studies of alkaline earth and alkali-alkaline earth oxides [9- 221. These work- ers have noted that the addition of alkali metals increases the basicity of the catalysts [lo] and consequently the selec- tivities to C,, hydrocarbons [9].

The present work compares the silica-supported alkaline earths prepared from either the acetates or chlorides of the latter with and without alkali metals as dopants and in the absence and presence of carbon tetrachloride (TCM). Earli- er work from this laboratory has demonstrated that the ad- dition of relatively small quantities of TCM to the methane feedstream produces enhancements of the conversions and/or selectivities with certain catalytic compositions [ 131.

2 Experimental

The catalysts were prepared by impregnation of silica (Grace-Davison, grade 407) with aqueous solutions of ei- ther the acetates or chlorides of the alkaline earths or the al-

0 VCH Verlagsgesellschaft mbH, D-69451 Weinheim, 1995 0930-751 6/95/0204-0132 $5.00+ .25/0

Chem. Eng. Technol. 18 (1995) 132-138 133

kali metals. The catalysts were dried in air at 110°C for 3 h. After installation in the reactor the catalysts were held in a flow of helium while the temperature was increased to 775 "C, following which a flow of oxygen (ca. 26 ml/min) was passed over the catalyst at the same temperature for 1 h prior to the commencement of the reaction.

The catalytic experiments were performed in a fixed-bed continuous flow reactor operated at atmospheric pressure. The catalyst was packed between two layers of quartz wool in the centre of the quartz reactor (an 8-mm I.D. and 35-mm long quartz tube sealed to 4-mm I.D. quartz tubes on two ends; the total length was 200 mm) to minimize the dead space with the aim of reducing the contribution of non-catalytic homogeneous reactions. The additive, tetrachloromethane (TCM), was admitted to the main flow of reactants, methane (Linde, CP; > 99.0%), oxygen (Linde, Extra Dry; > 99.6%), and diluent helium (Linde,

' Research: > 99.99%), by passing a separate stream of heli- um through a gas dispersion tube in a glass saturator con- taining the liquid at ice-water temperature. The standard conditions for the reaction were as follows, unless otherwise stated: W = 1.5 g, F = 30 ml min-', T = 7 7 5 " C , P(CH,) = 215 Torr, P(02) = 30 Torr, P(CC1,) = 1.1 Torr (when present).

The reactants and products were analyzed with a Hewlett- Packard 5880A gas chromatograph equipped with a TC de- tector and attached to an integrator. A Molecular Sieve 5A column (1.25 m x 1/8 in., 35 "C) was employed in the analy- sis of 02, CO and CH4. A Porapak T column (5.5 m x 1/8 in., 35 - 140°C) was used to analyze C02, C2H4, C2H6, C2H2, and C3 hydrocarbons. The columns were connected in series through a six-port VALCO switching valve to iso- late the MS-SA column while carbon dioxide and the hydro- carbons are eluted from the Porapak T column. The con-

Table 1. Conversion and selectivity on silica supported alkaline earth oxide and chloride catalysts (5 wt% metal) in the absence of TCMa).

Catalyst CH, Conv. Selectivity (To) (070)

CO COZ ClH, C2H6

Mg/SiOzb) 8.3 77.5 9.0 8.0 6.0 Mg/SiO ') 5.7 82.0 7.5 5.0 5.5 Ca/SiO)) 6.2 67.0 12.5 10.0 10.0

Sr/SiO:) 7.8 72.5 9.0 9.0 9.5 Ca/SiO ') 4.9 69.0 7.0 8.0 16.0

Sr/SiO,') 2.6 56.0 3.5 12.0 28.0 Ba/Si02b) 5.2 79.0 8.0 6.0 7.0 Ba/Si02c) 0.90 73.5 9.0 5.5 8.5

a) W = 1.5 g, F = 30 ml min-', T = 775"C, P(CH,) = 215 Torr,

b) Catalyst prepared from acetate of metal. c) Catalyst prepared from chloride of metal.

P(0,) = 30 - 32 Torr.

catalyst for which the combined C2 selectivities are a factor of two higher than those found with the strontium catalyst prepared from the acetate. However the largest C2 selectivi- ty reported in Table 1 is itself a factor of two lower than those generally reported in the literature.

Under the identical reactor operating conditions as em- ployed to obtain the results given in Table 1 but with 1.1 Torr of TCM present in the reactant stream, increases in conversion are evident for all the catalysts (Table 2). While the conversion increases by a factor of approximately two with the Ca, Sr and Ba catalysts prepared from the acetate, the increases are considerably larger when the correspond- ing chloride has been employed in the preparation. In addi- tion the selectivities to C2H4 are increased in the presence of TCM while those to C2H6 are generally decreased.

version of methane was calculated from the carbon-contain- ing products formed. The selectivities were calculated from the quantities Of products formed. These are ex- pressed as mole percent.

The addition of to the catalysts decreases the con- version in the absence of TCM but produces an increase in its presence (Table 3). The selectivities to C2 hydrocarbons

Blank experiments conducted in the absence of feedstream methane indicated that TCM undergoes oxidation produc- ing carbon monoxide and carbon dioxide. However, the quantities of carbon monoxide and carbon dioxide were negligibly small compared to those produced when methane was present in the feedstream. In any event, data were cor- rected for the contribution of TCM by performing blank ex- periments.

3 Results

The conversion and selectivities of the silica-supported al- kaline earth catalysts under similar conditions are com- pared in Table 1. With all of the alkaline earths the catalysts prepared from the acetates yield higher conversions under identical reactor operating conditions than do those pre- pared from the chlorides. In addition the selectivities to C2 hydrocarbons obtained under these conditions are lower on the latter catalysts with the exception of the strontium

Table 2. Conversion and selectivity on silica supported alkaline earth oxide and chloride catalysts (5 wt% metal) in the presence of TCMa).

Catalyst 112 h on time on stream 3 - 3 1 /2 h on time on stream

CH4 Selectivity CH, Selectivity (070) Conv. (070) Conv. (070) -

C2H4 C2H6 C2H4 C2H6

8.5 13.5 4.5 9.0 11.1 4.4 6.6 17.0 4.5 6.1 14.9 4.2 8.4 23.5 4.5 8.5 27.0 4.0

13.8 43.5 5.0 10.9 31.5 5.0 14.4 33.0 6.0 13.6 42.5 11.0 15.8 55.0 11.5 13.5 37.5 7.0 12.0 32.0 9.5 11.9 40.0 13.0

47.5 17.0 7.9 44.0 18.0 9.2

a) W = 1.5 g, F = 3 0 ml min-', T=775"C, P(CH4)=215 Torr,

b) Catalyst prepared from acetate of metal. c) Catalyst prepared from chloride of metal.

P ( 0 2 ) = 30-32 Torr, P(TCM) = 1.1 Torr.

134 Chem. Eng. Technol. 18 (1995) 132-138

Table 3. Conversion and selectivity on silica supported Li-doped alka- line earth oxide catalysts in the absence and presence of TCMa2 ').

Catalyst TCM') CH, Conv. Selectivity (To) ( 7 0 )

CO CO, C2H4 CZH,

Li-Mg Li-Mg Li-Ca Li-Ca Li-Sr Li-Sr Li-Ba Li-Ba

absent present absent present absent present absent present

0.65 17.2 5.2

14.1 4.8

19.2 5.2

21.3

65.0 t t 35.0 26.7 2.5 57.8 9.0 55.9 18.5 6.9 18.8 42.1 4.8 40.6 8.8 53.8 11.8 10.0 24.4 30.3 5.0 50.6 8.1 66.5 20.1 4.2 9.3 17.0 3.5 64.2 9.7

a) Catalyst composition: alkaline earth metal = 5.0 wt%, Li =

b) W = 1.5 g, F = 3 0 ml min-', T=775"C, P(CH,)=215-225

c) In the presence of TCM, conversion of 0, was greater than 90%.

1 .o wt%.

Torr, P(0,) = 32 - 35 Torr, P(CC1, = 1.2 Torr.

are also increased on addition of lithium but particularly so when TCM is present in the feedstream and in the latter case the unsaturated C2 hydrocarbon represents a much larger fraction of the product than the saturated species.

A comparison of Li-Mg/Si02 catalysts prepared by dif- ferent methods (see footnotes to Table 4) shows substantial differences in the absence of TCM but relatively little dif- ferences in its presence (Table 4). In the absence of TCM the conversions differ by as much as a factor of 5 while in the presence of TCM the conversions range from approximate- ly 13 - 17%. As before the selectivities to C2H6 are higher than those to C2H4 in the absence of TCM but the reverse is observed when TCM is present.

Table 4. The effect of catalyst preparation procedure, Li-Mg/SiO, cat- alysts (Li = 1.0 wt%, Mg = 5.0 wt%)a).

Catalyst TCM CH,~) Selectivity (970) Conv. ('3'0)

CO CO, C,H, C,H, C2+ totalc)

Cat. A absent 0.80 42.0 2.5 8.0 47.5 55.5 Cat. B absent 0.70 57.0 3.0 6.0 34.0 40.0 Cat. C absent 1.7 71.5 6.0 5.0 17.5 22.5 Cat. D absent 3.7 74.0 5.0 6.0 15.0 21.0 Cat. A present 16.2 30.0 2.0 58.0 5.5 68.0 Cat. B present 17.2 27.0 2.5 58.0 9.0 71.0 Cat. C present 12.8 30.5 13.0 46.0 8.0 56.5 Cat. D present 14.7 31.5 7.0 51.0 7.5 61.5

Cat. A Catalyst prepared by co-impregnation of lithium chloride and magnesium acetate.

Cat. B Catalyst prepared by co-impregnation of lithium acetate and magnesium acetate.

Cat. C Catalyst prepared by sequential impregnation: magnesium ace- tate first followed by calcination, then lithium acetate followed by calcination.

Cat. D Catalyst prepared by sequential impregnation: order of im- pregnation was reverse of C.

a) W = 1.5 g , F = 3 0 m l m i n - ' , P ( C H 4 ) = 2 1 5 Torr, P(0 , )=28-30 Torr, P(TCM) = 1.1 Torr when present.

b) When TCM was present, 0, conversion was greater than 95% with all catalysts.

c) C , , total includes small quantities of C,H, and C3's produced.

Only relatively small quantities of lithium are required to produce the aforementioned effects with the Li-Mg/Si02 catalysts (Fig. 1 ) . In the presence of TCM the conversion of methane increases up to a lithium content of approximately 0.5% and thereafter remains approximately constant. The C2+ selectivity has also reached a plateau at a similar lithium content.

Somewhat similar behaviour is observed for increases in the mass of the catalyst which may be related to increases in the apparent residence time. In the presence of TCM the selec- tivity to C2H4 and the conversion of methane appear to have reached constant values after the mass of the catalyst has reached approximately 0.75 g (Fig. 2).

The conversion of methane, in the presence of TCM, reaches a maximum, with a partial pressure of methane of 215 Torr, as the partial pressure of oxygen is increased to 90 Torr, while with 30 Torr of oxygen a maximum is at- tained as the partial pressure of methane is decreased to 100 Torr (Fig. 3). A conversion of approximately 28% is achieved in the latter case. In contrast the selectivity to C2 hydrocarbons increases to approximately 80% with a par- tial pressure of methane of 21 5 Torr on decrease of the par- tial pressure of oxygen to 19 Torr, and with a partial pres-

20 . , , , , , I . I 1 , . , .

-.-I

1 0

0 0.4 0.8 1.2 1.6

Lithium Content (wt%)

Fig. 1. Effect of Li content on the conversion of methane and selectivi- ties to products on Li-Mg/SiO, in the absence and presence of TCM. Lower figure: TCM present. Reaction temperature 775"C, W = 1.5 g, F = 30 ml min-', P(CH,) = 215 Torr, P ( 0 2 ) = 30 Torr, P(CC1,) = 1.1 Torr.

Chem. Eng. Technol. 18 (1995) 132-138 135

E

0 0.4 0.8 1.2 I .6"

Weight of Catalyst (g)

Fig. 2. Effect of apparent residence time on the conversion and selectiv- ities with Li-Mg/SiO, in the presence of TCM. Catalyst: 5.0 wt% Mg, 1.0 wt% Li. Reaction conditions as in Fig. 1 .

30 I

cH4 215 215 215 215 100 215 350 Torr O2 19 30 50 90 30 30 30 Ton

Fig. 3. Dependence of conversion and selectivities for Li (1.0 wt%)-Mg (5.0 wt%)/SiO, on the partial pressures of methane and of oxygen (TCM presenr). Reaction conditions as in Fig. 1.

sure of oxygen of 30 Torr as the partial pressure of methane is increased to 350 Torr.

Somewhat similar trends are found on addition of 10% so- dium to the various alkaline earth/Si02 catalysts as were observed on addition of lithium (Table 5). However a com- parison of Tables 3 and 5 shows that the addition of TCM is generally more advantageous where lithium is present. In- terestingly, although the selectivity to C2 + hydrocarbons increases with increasing sodium content and reaches a pla- teau at approximately 0.2%, the conversion of methane ul- timately decreases for higher sodium contents (Fig. 4).

A comparison of the effect of adding various of the alkali metals to a strontium/Si02 catalyst in the presence or ab- sence of TCM demonstrates that the largest enhancement in conversion on addition of TCM occurs in the presence of lithium while the largest beneficial effect on C2+ selectivity is found with the catalyst containing sodium (Table 6).

Table 5. The effect of TCM on oxidative coupling reaction on Na- doped alkaline earth oxide catalysts (silica supported)". "I.

Catalyst TCM') CH, Conv. Selectivity (To)

P J O ) CO CO, C,H, C,H,

Na-Mg/Si02 Na-Mg/SiOz Na-Ca/SiO, Na-Ca/SiO, Na-Sr/SiO, Na-Sr/Si02 Na-Ba/Si02 Na-Ba/SiO,

absent present absent present absent present absent present

0.45 2.3 1.6 6.7 0.52

14.0 0.24 6.1

52.4 8.9 4.7 17.0 18.4 7.2 46.4 18.8 32.5 7.6 10.2 49.7 14.8 4.0 55.8 18.0 53.6 3.4 4.6 38.3 14.3 2.4 60.8 17.7 45.0 t t 55.0 12.8 4.6 54.0 21.6

a) Catalyst prepared from acetates, alkaline earth metal = 5.0 wt%,

b) W = 1.5 g, F = 3 0 mlmin- ' , T = 7 7 5 " C , P(CH,)=215-225 Na = 1 .O wt%.

Torr, P(0,) = 30 Torr, P(TCM) = 1.1 Torr.

Table 6. The effect of TCM on oxidative coupling reaction on different alkali doped strontium oxide (silica supported)", ').

Catalyst TCM CH, Conv. (070)

Li-Sr/SiO, A 4.8'' Li-Sr/Si02 P 19.2')

Na-Sr/SiO, P 15.4 K-Sr/SiO, A 0.62

Na-Sr/SiO, A 0.55

K-Sr/SiO, P 4.4 Rb-Sr/SiO, A I .2 Rb-Sr/SiO, P 2.5

Selectivity ('70)

CO CO, C2H4 C,H,

53.8 11.8 10.0 24.4

53.6 3.4 4.6 38.3 16.2 3.1 62.8 12.9 65.5 5.5 3.1 25.2 14.5 2.7 51.1 24.6 74.0 8.7 3.6 13.7 15.1 5.2 50.0 25.0

30.3 5.0 50.6 8.1

a) Catalyst composition: alkali metal = 1.0 wt%, Sr = 5.0 wt%. b) W = 1.5g ,F=3Omlmin- ' , P(CH4)=215Torr, P ( 0 2 ) = 2 9 T o r r ,

c) P(0,) = 35 Torr. P(TCM) = 1.1 Torr.

136 Chem. Eng. Technol. I8 (1995) 132-138

a0

60

40

20

n 0 0.5 1 1.5 2 2.5

Na- Content (wt%)

Fig. 4. Effect of Na content on conversion and selectivities on Na (1 .O wtVo)-Sr (5.0 wt%)/Si02 in presence of TCM. Reaction conditions as in Fig. 1.

4 Discussion

It is commonly accepted that the scission of the carbon-hy- drogen bond in methane results in the formation of methyl radicals, although the mechanism through which this occurs remains in doubt [7]. The methyl radicals may combine with each other to produce ethane, one of the desired prod- ucts. However, methyl radicals may also form methyl peroxy radicals (CH300') which can be disadvantageously converted to deep oxidation products. Further, the C2 hy- drocarbons can undergo, with oxygen, conversion to car- bon monoxide and/or carbon dioxide. The decrease in se- lectivity to C2 hydrocarbons at a constant partial pressure of methane but an increasing partial pressure of oxygen can then be understood as a diversion of the methyl radicals from the favoured dimerization process and the further oxi- dation of the C2 hydrocarbons as the probability of these molecules encountering an oxygen molecule in the gas phase increases. Similarly, as the partial pressure of methane in- creases at a constant partial pressure of oxygen, the proba- bility of methyl radicals colliding with each other is en- hanced, resulting in an increase in the selectivity to C2 hy- drocarbons. Unfortunately, at least from a practical view- point the conversion of methane varies in reciprocal manner to that observed with the selectivity as the concentration of oxygen is increased. This results from the requirement for oxygen to serve (in whatever form) as the active site for

methane scission or to regenerate the depleted sites, presum- ably reduced by methane, on the surface of the catalyst.

In the absence of TCM silica-supported alkaline earth catalysts are relatively ineffective in the conversion of meth- ane and the production of ethane and ethylene regardless of the source of the alkaline earth. However, the source of the alkaline earth appears to influence the conversion, with those catalysts prepared from the acetate of the alkaline earth producing the higher conversion than those prepared from the chloride of the corresponding alkaline earth, al- though the values are small with all the catalysts regardless of the alkaline earth and its source. In general such differ- ences are not seen in the selectivities which are relatively un- affected by the anion in the alkaline earth salt. However the exception may be noted with the strontium catalyst which, when prepared from the chloride has a C2 selectivity ap- proximately twice as large as that observed with the catalyst prepared from the acetate. No systematic changes in either the conversion or selectivities with increase in the atomic number of the alkaline earth are evident.

With the alkaline earth/Si02 catalysts the addition of TCM to the feedstream produces changes which are dependent on the alkaline earth source, the nature of the alkaline earth and the time on stream. It is evident that as the atomic num- ber of the alkaline earth increases the enhancement of the conversion on addition of the TCM increases, although there is some decrease with barium. Further, whereas in the absence of TCM no trend in C2 selectivities with increasing atomic number of the alkaline earth was observed, with TCM present during methane conversion the selectivities in- crease with increase in the atomic number of the alkaline earth and, it follows, the enhancement effect due to the presence of TCM increases with the atomic number.

The addition of TCM to the feedstream generally produces increases in both the conversion and the selectivity, the lat- ter of which increases with time-on-stream with those catalysts prepared from the acetate of the alkaline earth, with the exception of magnesium. Evidently the effect of the addition of TCM is cumulative and consequently must be ascribed to a retentive interaction between the catalyst and the feed stream halogen compound.

The addition of TCM to the feedstream produces more dramatic changes with the lithium-doped alkaline earth/Si02 catalysts with which the conversions in all cases are increased by as much as a factor of 20. The selectivities to ethylene are also increased substantially, while those to ethane are decreased, except in the case of the lithium-bari- um catalyst. However, in all cases the total C2 selectivities are increased. It is clear that the interaction between TCM and the catalyst is strongly influenced by the presence of the alkali metal. Although the source of the advantageous re- sults from the addition of small quantities of lithium to the MgO catalysts is not yet understood, it has been suggested that, since the radii of Mg2+ and Li' are similar (66 pm and 68 pm, respectively), some substitution of magnesium ions by lithium ions may occur, leading to the creation of

Chem. Eng. Technol. I8 (1995) 132-138 137

0- radical centers which may serve as active centres for the methane activation process [ 5 ] .

Although the preparative method for the Li-Mg catalyst has a substantial influence on the conversion and selectivities in the absence of TCM, such differences are virtually eliminat- ed when TCM is contained in the feedstream. It should also be noted that the halogenated compound reduces the selec- tivity to CO while having, generally, relatively little effect on that to C02.

It is instructive to compare the changes produced on addi- tion of TCM in more detail for the various catalysts. For this purpose the increases in conversion and in the C2 selec- tivities are collected in Tables 7 and 8. It is clear from Table 7 that the presence of TCM produces higher increases in conversion with the alkaline earth catalysts prepared from their chlorides than those produced from the acetates. A re- lationship between the values for the conversion of methane and the radii of the alkaline earth ions is also evident in Ta-

Table 7. The increases in conversion of methane on addition of TCM to the feedstream.

~ a ) ,bl A Conversion M a)/Si02 Li-Mal/SiO,

Acetatec) Chloridec)

t , d ) r;) t , d ) t;)

Mg 65 0.2 0.7 0.9 0.4 16.6 Ca 99 2.2 2.3 8.9 6.0 8.9 Sr 113 6.6 5.8 13.2 10.9 14.4 Ba 135 6.8 6.7 8.3 7.0 16.1

a) M = alkaline earth. b) Ionic radius of alkaline earth (pm). c) Anion of alkaline earth salt employed in the preparation. d) f 1 = 112 h time-on-stream. e) t2 = 3 - 3 t /2 h time-on-stream.

Table 8. The increases in C, selectivities on addition of TCM to the feedstream.

Ma) ,b) A C, Selectivity Ma)/Si02 Li-Ma)/Si02

Acetatec) Chloridec)

t l d ) t2e) t ld ) t2@

Mg 65 4.0 1.5 11.0 8.6 31.8 Ca 99 8.0 11.0 24.5 12.5 23.7 Sr 133 20.5 35.0 26.5 46.5 24.3 Ba 135 28.5 40.0 50.5 48.0 60.4

a) M = alkaline earth. b) Ionic radius of alkaline earth (pm). c) Anion of alkaline earth salt employed in the preparation. d) f 1 = 1/2 h time-on-stream. e) t, = 3 - 3 1 /2 h time-on-stream.

ble 7. As the radius increases the enhancement of the con- version on addition of TCM increases although the trend is somewhat more ambiguous for the barium catalysts. It should be recalled that many atomic properties show peri- odic trends and the selection of ionic radii is therefore some- what arbitrary. It should also be noted that alkaline earth catalysts containing lithium show generally higher increases in conversion on addition of TCM to the feedstream than those where lithium is absent, and the systematic variation observed in the latter case is less evident than in the former.

The increases in C, selectivities (it should be recalled that the selectivities to ethylene and ethane generally increase and decrease, respectively) on addition of TCM, as with the conversion, again show a strong relationship with the size of the alkaline earth ion, regardless of the source of the alkaline earth (Table 8). Once again, with those catalysts containing lithium the aforementioned relationship is less pronounced.

Although it is difficult at this time to provide a rationaliza- tion of the aforementioned results a number of conclusions can be drawn. The addition of TCM to the methane conver- sion feedstream produces increases in the conversion and C2 selectivity with the alkaline earths as catalysts. The de- pendence of the enhancements on the nature of the alkaline earth provides strong evidence that the benefits of such ad- dition derive from the modification of the catalyst although participation of the TCM in any gas phase processes cannot be ruled out. The chlorine existing in whatever form in and on the catalyst, but probably as the chloride ion, appears to be participating in the C - H bond scission process. Since there is evidence from the present work that ethane is a pri- mary while ethylene is a secondary product, even in the presence of TCM, the enhanced production of ethylene can- not be attributed to a direct formation of such species.

It is at least tentatively believed that both the chloride ion and oxygen species on the surface of the catalyst are capable of serving as activating sites for the methane and the effec- tiveness of these, particularly the former is related to the charge density of the alkaline earth ion. The data presented here suggest that the effectiveness of the added chloride is inversely related to, at least in part, the polarizing power of the alkaline earth cation. Magnesium ions, with their rela- tively high polarizing ability, are less able to benefit from the introduction of the chloride than those alkaline earth ions with lower polarizing abilities. The chloride ion is thus more able to assist advantageously in the methane activa- tion process with catalysts prepared from alkaline earths of larger radii and, hence, lower polarizing capabilities.

Acknowledgement

The financial support of the Natural Sciences and Engineer- ing Research Council of Canada is gratefully acknowl- edged.

Received: January 18, 1994 [CET 6321

138 Chem. Eng. Technol. 18 (1995) 132-138

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