Energy & Fuels 1993, 7, 279-284
279
Anion Effects in the Oxidative Coupling of Methane on Salts of Lanthanum S. Sugiyamaf and J. B. Moffat' Department of Chemistry and Guelph- Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 Received July 27, 1992. Revised Manuscript Received December 7, 1992
The oxidative coupling of methane has been studied on three salts of lanthanum, the oxide, phosphate, and sulfate, both in the absence and in the presence of carbon tetrachloride (TCM) in the feed stream. The results show that the anion has a significant effect on both the conversion and selectivity to C2+ hydrocarbons. At 835 OC, for example, the C2+ selectivitiesand conversions follow the order Laz(SO4)s > La203 > LaPo4 and La203 > LaPo4 > Laz(SO4)3, respectively. The addition of TCM to the feedstream increases the conversion and the selectivity to C2H4 on Laz(SO4)a but decreases the conversion on LaPo4 and has little influence on these quantities with La203. Introduction During the decade which has elapsed since the pioneering work of Keller and Bhasin' on the oxidative dehydrogenation of methane was first published a wide variety of solids has been examined as possible catalysts for the methane conversion process. Of the many solids investigated those containing lanthanum frequently occur in reporta of such The earliest work on lanthanum oxide in which conversions of methane and selectivities to CZhydrocarbons were compared with various oxides showed that samarium oxide and lithium-doped samarium oxide were the most active of those studied with selectivities of 40 and 60%, respectively.u Somewhat later, a conversion of methane of 25% and a C2 selectivity of 47% were reported for lanthanum aluminum oxide.6p6 The addition of aluminum to lanthanum oxide was claimed to have a beneficial effect. Methane activation has also been studied on a series of
* To whom correspondence should be addressed.
+ Permanent address: Department of Chemical Science and Tech-
nology, The University of Tokushima, Minamijoeanjima, Tokushima, 770 Japan. (1)Keller, G.E.;B h i n , M. M. J. Catal. 1982,73,9. (2)Otsuka, K.; Jinno, K.; Morikawa, A. Chem. Lett. 1986,499. (3)Otsuka, K.; Lin, 8.;Hatamo, M.; Morikawa, A. Chem. Lett. 1986, 467. (4)Otauka, K.; Nakajima, T. Inorg. Chim. Acta 1986,120,L27. (5)Imai,H.; Tagawa, T. J. Chem. SOC.,Chem. Commun. 1986,52. (6)Imai, H.;Tagawa, T.; Kamide, N. J. Catal. 1987,106,394. (7)Campbell, K. D.;Zhang, H.; Lunaford, J. H. J. Phys. Chem. 1988, 92,750. (8)DeBoy, J. M.; Hicks, R. F. J. Chem. Soc., Chem. Commun. 1988, 982. (9)DeBoy, J. M.; Hicks,R. F. J. Catal. 1988,113,517. (10)Hutchina, G.J.; Scurrell,M. S.; Woodhouse,J. R. Catalysis Today 1989. ~ . .4. . 371. (11)Hutchings, G.J.; Woodhouse,J. R.; Scurrell, M. S. J. Chem. Soc., Faraday Trans. 1 1989,85,2507. (12)Choudhary, V. R.; Chaudhari, S. T.; Rajput, A. M.; b e , V. H. Catal. Lett. 1989,3,85. (13)Tong, Y.;Rosynek, M. P.; Lunsford, J. H. J. Phys. Chem. 1989, 93, 2896. (14)Tong,T.;Rosynek, M. P.;Lunsford, J. H. J.Catal. 1990,126,291. (15)Anshita, A.G.;Voskresenskaya,E. N.; Kurteeva, L. I. Catal. Lett. 1990,6,67. (16)Le Van, T.;Che, M.; Kermarec, M.; Louis, C.; Tatibouet, J. M. Catal. Lett. 1990,6, 395. (17)Aika. K.:- Fuiimoto. . N.:. Kobavaehi. - . M.:. Iwamatau. E. J. Catal. 1991,127,1.. (18)Wada, 5.;Imai, H. Catal. Lett. 1991,8,131. (19)Taylor, R. P.; Schrader, G. L. Ind.Eng. Chem.Res. 1991,30,1016.
.-,-
~
0881-0624/93/2501-0219$04.00/0
lanthanide oxides7 and on alkaline earth promoted lanthanum ~ x i d e .In~ the ~ ~latter case, the promotion with 1 wt % strontium was found to yield a CZselectivity of 72% and a conversion of 14% at 750 "C. Hutchings and co-workers also studied a number of oxides including that of lanthanum and have suggested that 0-is responsiblefor the production of C2 hydrocarbons while deep oxidation is attributed to a dioxygenspecies.'0J1 Choudhary and co-workershave shown that the addition of 5 % (molar) lanthanum to calcium oxide produces a methane conversion of 25 % with CZselectivity of 66 % .12 Lunsford et al. have shown that lanthanum oxide is effective in the formation of methyl radicals and is a poor radical 5~avenger.l~These workers have also studied the effect of the addition of sodium carbonate to lanthanide oxides, including that of 1anthan~m.l~ Anshita et al. have considered the role of defect structures of various oxides including that of lanthanum in the oxidative coupling of methane.ls Le Van et al. have demonstrated that the oxidative coupling of methane on lanthanum oxide is structure sensitive.16 Aika et al. have examined the effect of supporting various metal oxides including that of lanthanum on strontium carbonate.17 The partial oxidation of methane over fine particles of lanthanum oxide has been reported.18 Most recently, Taylor and Schrader have compared the reactivity of various lanthanum-containing phases such as are encountered in unpromoted lanthanum oxide and conclude that an oxycarbonate is the most active.le Work in this laboratory has recently been concerned with the effect of the cation in a series of phosphates and sulfates on the conversion of methane.20121It is of equal importance to provide information on the consequences of changesin the anion for agiven cation. Since lanthanum oxide has received considerable attention as a catalyst for the oxidative dehydrogenation of methane, lanthanum was chosen as an appropriate cation for the present study. In view of the evidence suggesting that effective catalysts for this process usually contain oxygen, frequentlyas the oxide, three anions containing oxygen, namely the phosphate, sulfate, and oxide itself were selected for the comparative (20)Ohno, T.; Moffat, J. B. Catal. Lett. 1991,9,23. (21)Sugiyama, S.;Moffat, J. B. Catal. Lett. 1992,13,143.
1993 American Chemical Society
280 Energy & Fuels, Vol. 7, No. 2, 1993
Sugiyama and Moffat
study. In addition, to complement recent work on the effect of the addition of gas-phase additives,22some of the present experiments have been carried out in the presence of small partial pressures of carbon tetrachloride in the feed stream.
20r
h
Experimental Section Laz(S04)3.9H20(99.999%, surface area 2.4 m2/g) and La203 (99.999 % ,surface area 2.1 m2/g)were obtained from Aldrich and used without further purification. LaPo4 was prepared by the addition of 25 mL of aqueous solution of La(N03)3.6H20 (9.32 g, 0.02 mol) to 30 mL of aqueous solution of (NHd)zHP04 (2.85 g, 0.02 The precipitate was dried at 110 "C for 2 h and calcined at 500 "C for 6.5 h. The catalytic experiments were performed in a fixed-bed continuous flow reactor operated under atmospheric pressure. The reactor was designed to minimize the free volume in the hottest zone to reduce the contribution of noncatalytic homogeneous reactions. The reactor consisted of a 9 mm i.d. and 35 mm in length quartz tube sealed at each end to 4 mm i.d. quartz tubes to produce a total length of 20 cm. Tubes of 7 or 4 mm i.d. were also used where appropriate for minimizing the dead space. The catalyst was held in place in the enlarged portion of the reactor by two quartz wool plugs. In those experiments in which carbon tetrachloride (TCM) or water was added, the additive was introduced to the main flow of reactants (CH4,02, and diluent helium) by saturating a separate stream of helium with TCM at 0 "C or H2O a t 25 "C, respectively. In all experiments, the temperature of the catalyst was raised to 775 "C while maintaining a continuous flow of helium and was then conditioned at this temperature under a 25 mL/min flow of molecular oxygen for 1 h. The reactor was then purged with helium and the reactor was adjusted to the reaction temperature before introduction of the reactant gas mixture of methane, oxygen, TCM (when present), distilled water (when present), and helium (total flow rate 30 mL/min unless otherwise stated). The reactor was charged with 1.4 g of catalyst except where the effect of residence time was studied. The reactants and products were analyzed with an onstream H P 5880 gas chromatograph equipped with a TC detector and integrator. Two columns, one Porapak T (18 ft X l / g in.) and the other Molecular Sieve 5A (35 cm X l / g in.) were employed in the analyses. The conversions and selectivities were calculated on the basis of the amount of reaction products formed as determined by the GC analysis.22 Blank experiments conducted with CHI absent from the feed (02+ He + TCM) indicated that TCM undergoes oxidation producing CO and/or C02. The data reported were corrected by running duplicate experiments with CH4absent under otherwise identical sets of values of the process variables.
60 50
:: 40
$
4
m N
30 20 10
0
s 80
La2 03
La PO4
La2(SOJ3
Figure 1. Comparison of catalytic activity on La203, and La2(SO4)3with additives (TCM and HzO) at 835 "C and 0.5 h on-stream. Condition: CHI 215 Torr, 02 30.4 Torr, TCM 1.3 Torr when present, and H20 10 Torr when present. Total flow rate 30 mL/min. Catalytic weight 1.4 g baaed on anhydrous catalyst. Pretreatment with 02 (25 mL/min) a t 775 "C. (a) Without additive. (b) With TCM. (c) With HzO.
The conversions,yields, and selectivitiesin the oxidative coupling of CHI on La203, LaP04, and La2(S04)3 at 835 "C and 0.5 h on-stream under our standard condition (CH$ 02 = 7.1) are shown in Figure 1. The main products on eachcatalyst were CO, CO2, C2H4, and (22% with or without additives (TCM or HzO). C3 species on La203 and C2H2, HCHO, and CH3C1(in the presence of TCM) on La2(SO4)3 were also detected but the amounts were small. Water was produced but is not reported here. On La2(S04)3, SO2 was detected, but was not analyzed. The effect of either additive was very small on La203 in contrast with LaPo4 and La2(S04)3. Methane conversion on LaP04 in the absence of TCM was relatively high and deactivation was not observed during 6 h on-stream (not shown), but
the major products were CO and COZ. In the presence of TCM, the conversion decreased significantly but little change in the selectivity was observed. In contrast, the Conversion of CH4, C2 yield, and C2H4 selectivity dramatically improved on Laz(SOJ3 in the presence of TCM. On each catalyst, the introduction of water did little to change the observed reaction behavior, in contrast to the reports in the literat~re.~3 The remainder of the report focuses specifically on the results obtained with La203 both in the presence and in the absence of TCM and La2(SO& in the presence of TCM in the feed. The effect of temperature on the reaction on Laz(SO4)s in the presence of TCM is shown in Figure 2. On La2(SO&,, the conversion of CHI, C1 selectivity, and CZ selectivity change relatively little with temperature below 800 "C. For temperatures of 775 "C and less, an increase of the time-on-stream from 0.5 to 6 h produced significant decreases in conversion but relatively little changes in the C1 and C2 selectivities, while at 835 "C the conversion and C2 selectivities both increased. The formation of COO increased with time on-stream at temperatures of 800 and 835 "C. It is of interest to note the rather substantial selectivities to CH&1 at 700 OC and the decrease in these values as reaction temperature is increased, so that by 835 "C essentially insignificant quantities of CH3C1 are observed. At the lower temperatures studied, the selectivity to CH3C1is increased with increase in the time on-stream. These observations suggest that CH3C1may be converted to CO, particularly at the higher temperatures and consequently CHsCl may not be an intermediate in the production of C2 compounds. However, both C2 and C1 selectivities are relatively constant with change in reaction temperature. With LazO3, the conversion of methane increased with increase in reaction temperature and with
(22) Ahmed, S.; Moffat, J. B. J. Catal. 1990, 125, 54 and references therein.
(23) Pereira, P.; Lee, S. H.; Somorjai, G . A.; Heinemann, H. Catal. Lett. 1990, 6, 255.
Results
______
Oxidative Coupling of Methane
Energy 6 Fuels, Vol. 7, No. 2, 1993 281
'F
12
::- o Y
161
CO2
20
CO
E 8 1 ' 6
700'C
725'C
750'C
775.c
8002
835'C
Figure 2. Temperature effect on La~(S04)3 with TCM. Condition: same as those in Figure 1. (a) 0.5 h on-stream. (b) 6 h on-stream. 121 10
LO
A"I Cow Yield
CHL/02
Figure 3. Effect of C)4/02 ratio on La*(SO&with TCM at 750 O C . Condition: same as those in Figure 1 except partial pressure of C& and 0 2 . (CHI 106 Torr and 0 2 30.4Torr at CHdOz = 3.5, C& 215 Torr and 0 2 = 30.4 Torr at CHJOz = 7.1 and CHr 215 Torr and 02 = 15.2 Torr at CHJOz = 14.2.) (a) 0.5 h on-stream. (b) 6 h on-stream. the introduction of TCM (not shown). In general, the C2 selectivities increased while those to CI decreased with increase in temperature, but above 750 "C the C2 selectivities decreased, a t least in the absence of TCM. As expected, with increasing CH4/02 ratio, the CH4 conversion at 750 "C decreased on Laz(SO& and the CZ selectivity increased, the latter due largely to an increase in C2H6 selectivity (Figure 3). However, the deactivation behavior was found to vary irregularly with the CH4/02 ratio. Thus, the conversion of methane decreased to approximately half its value with an increase in time on-
*
z!
4
"
O
2 1.17
2.34 4.67 7.00 lo2 W / F / g min. m?
Figure 4. Effect of W/Fon La&04)3 with TCM at 750 O C . Condition: same as those in Figure 1 except catalytic weight (see values in Figure 4). (a) 0.5 h on-stream. (b) 6 h on-stream. stream from 0.5 to 6 h for a CH4/02 ratio of 7.1 while showinglittle change at ratios of 3.5 and 14.2. Significantly, the selectivity to CH3C1 approximately doubles for the same increase in time-on-stream at the CH4/02 ratio of 7.1. In contrast to the observations with La2(S04), the measured quantities with La203 a t 750 "C vary more regularly with changes in the CH4/02 ratio (not shown). The conversion of methane and the C1 selectivity decrease while the CZselectivity increases with increasing CH4/02 ratio. The addition of TCM increases the conversion and C1 selectivities but not enormously while decreasing the selectivity to CZhydrocarbons. Changes in the time onstream from 0.5 to 6 h have little or no effect on the measured quantities. The effect of residence times with Laz(SO4)s at 750 "C in the presence of TCM is shown in Figure 4. Between 102W/F = 1.17 and 4.67, the conversion of methane after 0.5 h and the selectivity to CO and C02 increased while that to C2 hydrocarbons decreased, the latter largely as a result of the decrease in C2H6, as the residence time increased. Somewhat surprisingly, the aforementioned quantities were approximately the same at 102W/F = 4.67 and 7.00. It is interesting to note that the selectivity to CH3C1after 0.5 h time on-stream decreased almost linearly from approximately 10% at 102W/F = 1.17 to approximately 1% at 102W/F = 7.00. After the longer time onstream, the conversion has decreased but the selectivity to methyl chloride has increased at all values of W/F examined. In contrast to the results with Laz(SO& little or no effect of W/F is observed on La203 regardless of whether W or F was changed (not shown). It appears that the oxidative coupling is completed in the inlet portion of the catalyst due to complete consumption of 02. The concentration of TCM in the feed stream has a marked effect on the conversion of methane on La2(S04)3
Sugiyama and Moffat
282 Energy & Fuels, Vol. 7, No. 2, 1993 a
IO $ 5 2 0 6 $
9 8 7 6 5
!
L
.o
9 'I
3
u
2
2
1
n
in
zE
o 60 ", LO 20
11
g
2 f
$
I1
45-
60 LO
-
20 s 2 0
Z 60 '??
40
s 20 F n %
2
V
3c 5 p% '0
uv)
t'~a b
d
0.~5
I ~5w~*/.LacI,/Lal(so,)~
TCM Concentration/ Torr
Figure 8. Effect of concentration of TCM in feed on Laz(SOd3 and comparison with reaction behavior on 5 w t 7% LaCldLaZ(SO& without TCM at 750 OC. Condition: same as those in Figure 1 except partial pressure of TCM (see values in Figure 5). (a) 0.5 h on-stream. (b) 6 h on-stream.
(Figure 5). The conversion increases from approximately 0.5% in the absence of TCM up to 10% with a partial pressure of TCM equal to 2.6 Torr in the reactant stream. While at the same time the selectivity to C2 hydrocarbons is little changed, the introduction of TCM increases C2H4 to the detriment of C2H6. However, a maximum appeared in the C2 selectivity at the standard conditions (TCM = 1.3 Torr) of the present work. Methyl chloride begins to appear only at the higher partial pressures of TCM. For the purpose of comparing the introduction of a gaseous chlorinated species with one containing chlorine in the catalyst, a 5 wt % LaCls/La2(S04)3catalyst was examined in the absence of TCM (Figure 5). While the conversion obtained was higher than that observed for Laz(SO& in the absence of TCM, the value obtained was considerably smaller than found in the presence of either 1.3or 2.6 Torr of TCMwithLaz(S033. WithLa203thereisnosystematic variation in the measured quantities with increase in the pressure of TCM (not shown). The addition of 5 w t % Lac13 to La203 produces relatively little change from the results obtained with La203 alone either in the presence or in the absence of TCM. The effect of the introduction of TCM is compared to that of CHsCl over La2(S04)3 at 750 and 835 "C (Figure 6). At either temperature the addition of CH3Cl has a qualitatively similar effect as TCM, althoughthe increases in conversion are considerably greater with the latter. When only CH3Cl and 02 were passed over either La2(SO4)or La203 under identical process variables as those used in generating the data of Figure 6, the major product was CO, with little or no evidence of C2 hydrocarbons (Table I). A comparison of the effect of the introduction of either CHsC1 or TCM to the feed with La203 as the catalyst demonstratesthat, as with TCM and this catalyst, CH&1 has relatively little influence on the resulta (not shown).
o b
\\
"U
'?
NO CH,CI Additive
TCM
Nb CGI
Additive
a b
FCM
Figure 6. Comparison of reactivity with C1 additives (TCM, CH3C1, and no additives) on La2(S0d3 at 750 and 835 O C . Condition: same as those in Figure 1 except partial pressure of CHsCl (1.52 Torr). (a) 0.5 h on-stream. (b) 6 h on-stream. In order to examine the role of the catalyst in the presence of TCM, experiments were performed in which La2(S04)3was exposed to TCM diluted with helium prior to introduction of the reactant stream, which did not contain TCM (Figure 7). It is evident that the conversion of CHI (1.4%), after the pretreatment of the catalyst in TCM, was higher than that observed without the pretreatment (0.5%)even though TCM was absent from the reactant feedstream. However, it is apparent that the conversion and selectivity both decrease as the time onstream increases. Following a second pretreatment, this time with both TCM and 02, the conversion of CH4, with TCM absent from the feed, was similar to that observed in the absence of any pretreatment, but the selectivity to CZhydrocarbons increased by a factor of approximately 2. On subsequent introduction of TCM to the feed, the conversion of methane increased to approximately 2% while the CZ selectivity exceeds 60%. These results demonstrate that the catalyst is participating in the mechanism involving the chlorine species. It is also of interest to examine the oxidation of C2Hs and C2H4 over La203 and La&04)3 in the presence and absence of TCM (not shown). With La2(S04)3 the conversions of C2H4 and of C2H6 were increased by the addition of TCM while the CZ selectivities remained relatively unchanged. With Lap03 the conversion of C2H6 increased in the presence of TCM while that of C2H4 was essentially unchanged. With La203 the selectivities were relatively invariant. Discussion
The conversion of methane as well as the selectivities of CzH.4,C2H6, CO, and C02quite evidently are dependent on the nature of the anion. The conversions,in the absence of any additive, follow the order La203 > LaPo4 > La2(504)s while the selectivities of C2H4 and C2H6 are in the order La2(SO4)3> La203 > LaPo4. Further, the effect of the addition of TCM is also dependent on the nature of
Energy & Fuels, Vol. 7, No. 2, 1993 283
Oxidative Coupling of Methane Table I. Comparison of Activity for CH&l t CH3C1 conv, %
02 conv, % 34
0 2
on Laz(SO& and La203 at 750 OC and 0.5 h On-Streama Selectivity ( % ) of
co
CHI
c02 5.06
CHGH
CZ+yield, %
Laz(SOd3 99.5 90.9 1.10 2.91 La203 100 75 100 CH3Cl = 15.2 Torr; 0 2 = 30.4 Torr; total flow rate 30 mL/min; W = 1.4 g (based on anhydrous catalyst).
Time-on-Stream/hr
Figure 7. Pretreatment effect with TCM on La2(S0& Pretreatment A TCM (1.3 Torr) diluted with He (total flow rate 30mL/min); reactant A: same as those in Figure 1 in the absence of TCM. Pretreatment B: TCM (1.3 Torr) and 0 2 (30.4 Torr) diluted with He (Total flow rate 30 mL/min); reactant B: same as those in Figure 1 in the presence of TCM. Filled symbols: with pretreatment. Open symbols: without pretreatment.
the anion with enhancement of the conversion being largest with the sulphate, relatively minor on the oxide and negative on the phosphate. Only minor changes are evident with the selectivities to C2 hydrocarbons. Introduction of water has little effect on either conversion or selectivity with any of the anions, in contrast to reports in the literature with other catalysts.23 With La2(S04)3, increase of reaction temperature from 700 to 800 "C increased the conversion of methane as measured after 0.5 h on-stream relatively little. However, an increase from 800 to 835 "C increased the conversion by greater than 50 % . For 6 h time on-stream the effect of temperature was greater, partly due to the fact that at the lowest temperature, for example, the conversion had decreased to half the value found at 0.5 h. It is interesting to note that the deactivation of the catalyst was less rapid at the higher temperatures and at the highest temperature the activity increased with time on-stream. At the lowest temperature the selectivity to CH3C1 is quite significant, but this decreases to a negligiblevalue at 835 "C. Increase in time on-stream at the lower temperatures increases the selectivity to CH3C1. Thus, there appears to be an approximately inverse correlation between conversion of methane and selectivity to CH3C1. Since the conversion is expected to be directly related to the ability of the catalyst to activate methane, it appears that CH3C1 may be acting as an intermediate to methyl radicals, assuming that these are the products of the dissociation of methane and that the dissociation of the CH3C1 is strongly temperature dependent. However, since there is evi-
2.90 0
d e n ~ that e ~ the ~ rate of production of C2 relative to that of COX is proportional to the concentration of methyl radicals, the observation that the selectivities to C2 and COXproducts are relatively constant with temperature appear to be contradictory. The change in activity of La2(S04)3 with reaction temperature is probably the result of loss of SO2 which would increase with reaction temperature producing more La203 in the catalyst which has a higher activity than the sulfate. The observations that the conversion of CH4 increases while the selectivity to CH3C1 decreases with residence time provides support for the previous remark, that CH3C1 appears to be an intermediate, but of course not necessarily the only one, in the production of methyl radicals. However, unlike the observations with increasing temperature with which little or no changes in selectivities to either C2 or COXwere evident, these selectivities do change somewhat with increasing residence time. Recently, B u r ~ has h ~provided ~ a comparison between samarium oxide and samarium oxychloride and has reported that the catalytic activities are similar. Although it is, of course, quite possible that lanthanum oxychloride is being formed from the reaction of TCM with the present catalysts, it seems difficult to explain the differences in the effect of TCM with the three anions on this basis. Although relevant thermodynamic data are not available, it would be expected that the formation of the oxychloride would not be dependent on the anion. It is clear that the conversion of methane and the selectivities to the various products are dependent on the anion both in the presence and absence of TCM in the feedstream. While there appears to be general agreement that an effective catalyst for the methane conversion process necessarily contains oxygen, the nature of the species of oxygen remains uncertain.26 In addition, it apears to be commonly accepted that the principal function of the catalyst is concerned with the activation of methane. It may then be assumed, not unreasonably, that the ability of the catalyst to bring about the scission of the C-H bond in methane is dependent upon the magnitude of the negative charge on the oxygen atoms of the solid. Although accurate data on such partial charges are not available for the present materials, values may be estimated by the semiempirical electronegativity equilization methoda27 Values obtained for oxygen in La203, LaPO4, and La2(SO& are -0.49, -0.30, and -0.21, respectively, which corresponds to the order followed by the methane conversions. Although the selectivities to C2+ hydrocarbons will obviously be dependent on a number of factors related to (24) Roos, J. A.; Korf, S.J.; Veehof, R. H. J.; Van Ommen, J. G.; Ross, J. R. H.Appl. Catal. 1989,52, 131. (25) Burch, R.; Chalker, S.; Loader, P.; Rice, D. A.; Webb, G. Appl. Catal. 1991, 79, 265. (26) Amenomiya, Y.; Birss, V. I.; Goledzinowski, M.;
Galuszka, J.; Sanger, A. R. Catal. Rev.-Sci. Eng. 1990,32,163. (27) Sanderson, R. T. Chemical Bonds and Bond Energy; Academic: New York, 1976.
284 Energy
I%
Fuels, Vol. 7, No. 2, 1993
both surface and gas phase processes, as a first-order approximation it may be suggested that more labile oxygen atoms should lead to undesirable, that is, oxygenated products. Semiempirical calculations provide values of the polar covalent energies for the oxygen bonds in Lap03, LaP04,and Lap(SO& as 212,98, and 89 kcal/mol, respectively. Although the selectivities follow the order for the first two of these catalysts, the sulfate is displaced from its expected position. The differences in behavior of CH3C1 and TCM as additives may be attributed, at least in part, to the differences in decomposition temperatures and the quantity of chlorine available from these two species. Since there is strong evidence that chlorine-containing species are formed on the catalyst, it is expected that TCM should be more effectivein such surface modifications. However, CH3C1is expected to be capable of the production of methyl radicals, a process which is obviously not possible with TCM. It is important to note that the decomposition (either thermal and/or catalytic) of CH3Cl and TCM
Sugiyama and Moffat
increase, not surprisingly, with temperature. This is undoubtedly a contributing factor in the relatively small temperature coefficientobserved in the present work. The conversion of Cp hydrocarbons to CO,, which is expected to increase with increase in temperature, and hence the expected decrease in selectivity to Cp hydrocarbons may be relatively small as a consequence of the opposing production of methyl radicals from CH3C1 and the generation of chlorinated surface species from TCM. Thus the relatively small temperature coefficient may be attributed to the interplay of a number of opposing factors. However, both results from earlier and the present report provide strong evidence for the direct participation of the catalyst in the effects of addition of TCM as an additive to the feedstream. Acknowledgment. The financial support of the Natural Sciences and Engineering Research Council is gratefully acknowledged.
