- Email: [email protected]
Measurement of kinetic isotope effects over methane coupling catalysts in the presence of carbon dioxide
M. de Pontes, R.L. Espinoza, C.P. Nicolaides, J.H. Scholz and M.S. Scurrell (Editors) 355
Natural Gas Conversion IV
Studies in Surface Science and Catalysis, Vol. 107 9 1997 Elsevier Science B.V. All rights reserved.
Measurement of kinetic isotope effects over methane coupling catalysts in the presence of carbon dioxide. Noel W. Cant a, Peter F. Nelson b and Bronwyn L. D u f f y b School of Chemistry, Macquarie University, NSW 2109, AUSTRALIA CSIRO Division of Coal and Energy Technology, PO Box 136, North Ryde, NSW 2113, AUSTRALIA 1. I N T R O D U C T I O N Despite a decade of study, details of the mechanism of the catalytic oxidation of methane to C2 hydrocarbons remain uncertain. The original proposal of lto et al. (1) for Li/MgO catalysts was as follows [Li+O -] + CH 4 ----> [Li§
-] + CH 3
C H 3 + C H 3 ----ff C2H 6
2[Li§ -] ---> [Li§ -] + Li§ + H20 [Li+O2l + Li+[] + 89 ----> 2[Li+O-I
[ll [21 [31 [41
with step [1] fast and [3] or [4] rate limiting (where [] is a surface vacancy). However this is inconsistent with the observation that CD4 reacts considerably slower than CH4 (2,3). This deuterium kinetic isotope effect (KIE = rate(CH4)/rate (CD4)), which has also been demonstrated for other catalysts (4), implies that CH bond breaking, and hence step [ll, is the rate determining step. Experiments comparing the rate in the presence of H20 or D20 suggested that step [3] is unlikely to be rate limiting (2). Two recent reinvestigations of the phenomena with Li/MgO catalysts have reached somewhat different conclusions (5,6). Shi et al. (5), in experiments with the methane concentration held at 25%, reported that the KIE value dropped when the oxygen content was reduced from 25% to 2.5%. Modelling led to rate parameters which were consistent with the kinetic orders observed by Roos et al. (7) in experiments with CO2 in large excess, and also with the observed variation in KIE. However Cant, Kennedy and Nelson (6) found that this data set was inconsistent with both kinetic observations made in the absence of added CO2 and the constancy of the KIE in experiments with varying methane concentrations. These findings, coupled with the extent of isotope mixing in experiments with ~6Oj~802 (8), led to an alternative data set which reaffirmed [1] as the limiting reaction. One possible explanation for these discrepancies is that at the lowest oxygen concentrations used by Shi et al. (5), inhibition by CO2 brings the rates of steps Ill and [4] closer together and this affects the measured KIE. The present study was designed to test this possibility through a determination of both the KIE, and the ~602/~gO2 mixing rate, in the presence of carbon dioxide. Some of the experiments used ~3CO2 in order to allow the extent of total oxidation of methane to be assessed even though the added carbon dioxide is much in excess of that produced by reaction. 2. E X P E R I M E N T A L The experimental procedures were similar to those described previously (3,6,8). The catalyst samples (200mg) were packed in a 4mm ID fused alumina tube between upper and lower thermocouple wells of 3mm OD and heated in a tube furnace. The standard reaction mixture was supplied by three mass flow controllers (Brooks Inst.) delivering CH4 (Matheson UHP grade), an analysed 12.5%O2/He mixture and diluent helium (CIG UHP grade). Additional mass flow controllers provided CD4, a
356
10%1sO2/He mixture and either ~2CO2 o r 13CO2. The CD4(>99%D) and the ~sO2 (>98%180) were obtained from CIL. The CD 4, unlike that used previously (2,3,6), was sufficiently pure that no correction of the product analyses for the presence of higher hydrocarbons was necessary. The ~3CO2, obtained from MSD Isotopes, was of high 13C purity (>99.3%) but it did contain, as noted later, some ~3cl6olSo. The product mixture from the reactor, or the reactants on bypass, were sampled by a two column gas chromatograph and a quadrupole mass spectrometer (VG model SX200) and then passed through a 10cm gas cell. The contents of this were analysed off-line by a Digilab model FTS20/80 PTIR with 256 scans of resolution 0.25 cm ~ collected for each spectrum. The present experiments used a new batch of catalyst made by the procedure described by Edwards et al. (9). Its lithium content after an initial calcination for 8 hours at 900~ was 0.2 wt% and the surface area 0.2mE/g. All kinetic experiments were carded out with conversions of the limiting reactant (02) held below 30% to allow calculation of rates using the differential reactor approximation. 3.
R E S U L T S AND DISCUSSION
Fig. 1 shows the effect of added 13CO2 on the rate of methane oxidation to each product in experiments using 50%CHJ5%O2 at 728~ Formation of all products except carbon monoxide are reduced to a significant degree. However, the EC2 selectivity remains constant at = 80%. The apparent reaction orders in carbon dioxide as calculated from the slopes of log-log plots are shown in Table 1. Formation of ethane and CO2 are inhibited to similar extents with apparent orders of-0.47 and -0.54 respectively. Production of ethene and C3 compounds are affected more (orders of-0.70 and -0.95) as expected since they are secondary and tertiary products respectively. The effect on carbon monoxide production is much less (order-0.1) probably because a reduced rate of formation from methane is partially compensated by additional production by the reverse water gas shift reaction between co-product hydrogen and added 13CO2 C O / + H 2 ---) CO + H20
[5]
to produce ~3CO as demonstrated by the FTIR spectrum in Fig. 2B. However this reaction is far from equilibrium. The 13C/~2C ratio in the carbon monoxide (= 0.8 from Fig. 2B), is very different to that in
,n. o
10
I
I
9 \
x
~"
~X
8 -
s
_
~
~ \
E
Table 1 Apparent reaction orders in CO2
1
9
CO
9
CO 2
9 A 9
Ethene Ethane C3's
-
Product C2H 6 COz C2H 4 Y'.C3 CO
B
'10 t~ L-
o G)
0 0
1
2
3
4
Average 002 Concentration (% v/v) Figure 1 728~
Effect of ~3CO2 on product formation at
order -0.45 a, _0.54 a -0.70' -0.95" -0.09 a
-0.41 b _c -0.69 b -0.69 b 0.07 b
' At 728~ with 50%CH4/5%Oz at 30 cm3/min, 200 m g catalyst. b At 778~ with 20%CH4/10%O2 at 40cm3/min, 200 mg catalyst. c ~3CO~ not used.
357
1.6
12002 ~ 13CO2
1.2
A
.8 .4 m 0.0 o r=
720
700
680
660
640
620
600
.04 .03 .02 .01 0.00
I
I
I
I
I
2200
2150
21 O0
2050
2000
Wavenumbers
(r
"1)
F i g u r e 2 FTIR spectra of CO and CO2 from reaction of 50%CH4]5%O2/2.5%13CO2 at 728~ the carbon dioxide (= 14 from Fig. 2A and product analyses). Fig. 2A also shows an additional Q branch at 645cm 1, attributable to 13C160180, which was found to comprise =7% of the starting 13CO2. Experiments to determine the effect of carbon dioxide on the 1602/1802 mixing reaction 1602 "t" 1802 It~ 2160180
[6]
were therefore carried out with 12CO2 rather than 13CO2 added as the 180 in the latter would have complicated the analyses unduly. Fig. 3A shows the 1602, 160180, 1802 distribution with no added CO2 while Fig. 3B shows the effect of CO 2 addition on the extent of mixing (the fractional approach to equilibrium), X m, defined as X m = [(16O180)out - (160180)in]/[(160180)** - (160180)in ]
[71
It is apparent that the exchange reaction is inhibited to a much greater extent than was production of ethane and carbon dioxide (Fig. 1). X m has a value of 0.30 in the absence of added CO2 but this falls to 0.017 when the average CO2 concentration reaches 4.75%. A log-log plot of the data gave an apparent order of -1.1 in CO2, double that for the inhibition of ethane production in Table 1. It may be noted that X m reached 0.94 in an experiment using 1602/1802/He alone (ie without methane) under the same conditions. Thus the CO2 produced by coupling alone is able to drastically obstruct the oxygen mixing reaction. (The reduction in Xm from 0.94 to 0.30 is equivalent to reduction in rate by a factor of almost l0 assuming that the isotope reaction is first order in the distance from equilibrium). Methane coupling and oxygen exchange may occur on different types of surface sites, however, the greater effect of CO2 on isotope mixing compared to methane coupling can be readily rationalised as follows. In terms of a single oxygen molecule, equation [4] is
358
2[Li+O 2] + 2Li+[] + 02 --~ 4[Li+O -]
[81
The forward reaction yields 2 O which may initiate methane coupling via hydrogen abstraction. However reversal of reaction [8] does not, on its own, produce isotope mixing. That requires dissociation of 1602 and 1802 molecules on adjoining sites and their random recombination. Thus the site requirement for mixing is larger than that for methane activation. If carbon dioxide blocks a large fraction of the surface, as seems likely, then some of the remaining sites may still be able to achieve oxygen dissociation, and then methane oxidation, but not oxygen isotope mixing. As a further consequence it is not possible to accurately estimate the true rate of oxygen recombination (the reverse of step [4]) from the amount of 160~80 evolved when much CO2 is present, but only a lower limit to it. Additional amounts of 1602 and 1802 may dissociate on isolated sites where recombination cannot give rise to isotope interchange. Although the oxygen isotope mixing reaction, [6], is far from equilibrium (Fig. 3A) and still further from it with added CO2 present (Fig. 3B) the three isotopic carbon dioxides in the exit gas are fully equilibrated as illustrated by the FTIR spectra of Fig. 4A. If the CO2 produced by reaction was simply additional to that introduced as 12CO2, then the distribution between C1602 , C160180 and C~802 would have been 0.80:0.13:0.07. However, the observed distribution was 0.75:0.23:0.02 which agrees with the equilibrium one within the experimental error as shown in Fig. 4B. Table 2 summarises the results of experiments to determine the deuterium kinetic isotope effect (KIE) in the presence and absence of added CO2 under two sets of conditions. The pattern of KIE values, and the overall effect (1.5 to 1.6) is similar to that reported previously (2,3,6). Shi et al. (5), in experiments with no added CO2, reported lower values when using high CHJO2 feed ratios. This is not apparent in the present data even though the 5 0 % C H J 5 % O 2 mixture used in the experiment at 728~ has the same ratio as the highest one used in their work (25%CHJ2.5%O2). As may be seen from Table 2, the presence of 1.5%13CO2 at 769~ with a CHJO2 ratio of two, has little effect on the distribution
359
of KIE values. It is possible that a small reduction in the KIE to C2H 6 and EC2 occurs when 2.5%13CO2 is added to the 50%CH4/5%O 2 feed at 728~ but the accuracy of these measurements is somewhat less due to low conversions. Unfortunately the determination of 12CO2by FTIR under these conditions was too inaccurate to provide a reliable estimate for the KIE to this product and therefore for the overall reaction. Overall it appears that the rate determining effect retains a large component of carbon-hydrogen bond breakage when a large excess of CO 2 is present even though this interferes with oxygen dissociation/recombination, as revealed by oxygen isotope mixing (Fig. 3B) and induces a different kinetic regime (7). While no exact determinations of the kinetic orders in CH 4 and 02 in the presence of CO 2 were attempted here, two-point estimates indicated that they approached those expected from the data of Roos et al. (7), first order in O 2 and zero order in CH4, and were considerably different from
Table 2 KIE values for methane coupling with and without added '3CO2
% 13C02
KIE = rate(CH4)/rate(CD4)
Conditions
added
C2H 6
C2H a
CO 2
CO
EC 2
overall
20% methane, 1 0 % 02 769~ 50cm3/min.
nil 1.5
1.48 1.54
2.2 2.7
1.25
1.47
1.1 1.3
1.66 1.65
1.58
50% methane, 5% 02 728~ 30cm3/min.
nil 2.5
1.50 1.38
2.4 2.6
1.17 -
1.0 1.2
1.61 1.45
1.51
1.51 -
360
those found previously (6) for reaction in the absence of added Table 3 CO2 (0.3 and 0.7 respectively). Oxygen atom recombination relative to methane On the basis of measurements consumption using 50%CH4/2.3%1602]2.3%1802 at with 90%CH4/5%1602/5%lsO2 728~ mixtures (8), and modelling for other conditions (6), we have previously concluded that the % CO 2 Xm ECH4 O....>O2]CH4 a number of oxygen atoms which added (loss,%) (loss) undergo dissociation and recombination to oxygen molecules, nil 0.46 3.1 2.2 ie step [4] and its reverse, is 2 to 20 times as great as those which attack 2.2 0.045 1.0 0.8 methane. Table 3 repeats this calculation for experiments using 50%CH4/2.3%I602/2.3%lsO 2 mixtures and carded out as part of " equals 4Xm[O2]~CH4(loss ) Ref.(6) the sequence to determine the KIE at 728~ In the absence of added CO2 the rate ratio falls at the lower end of the above range as expected for conditions when the methane concentration is high and the oxygen concentration low (6). With 2.2% CO2 present the calculated rate ratio does falls below unity because 160180 is inhibited more than methane. However, as explained in connection with the data of Fig. 3 previously, this is only a lower limit since the 1601sO formation rate does not provide a good estimate for the rate of the reverse of step [4] when CO2 occupies a large fraction of the surface. 4. CONCLUSIONS (1) The present results indicate that the oxygen species which attacks methane in the coupling reaction is the same as that involved in the oxygen isotope mixing reaction. The greater effect of carbon dioxide on the latter reaction can be explained in terms of its larger site requirement. (2) A substantial deuterium kinetic isotope effect occurs in the presence and absence of CO 2 indicating that CH bond breaking is rate limiting in both situations. There may be a slight reduction in the KIE when using both a high CHJ02 ratio and a large excess of CO 2 but the data is insufficiently accurate to establish this with certainty. (3) Finally while the present results are readily interpreted in terms of the model described by steps [1] to [4], which is based on the existence of surface O species, this should not be taken as proof of it since similar models based around other oxygen species are possible. 5. R E F E R E N C E S 1.
T. Ito, J.-X. Wang, C.-H. Lin and J.H. Lunsford, J. Am. Chem. Soc., 107 (1985) 5062.
2. 3. 4. 5. 6. 7. 8. 9.
N.W. Cant, C.A. Lukey, P.F. Nelson and R.J. Tyler, JCS Chem. Comm., (1988) 766. P.F. Nelson, C.A. Lukey and N.W. Cant, J. Catal., 120 (1989) 216. L. Lehmann and M. Baems, J. Catal., 135 (1992) 467. C. Shi, M. Xu, M.P. Rosynek and J.H. Lunsford, J. Phys. Chem., 97 (1993) 216. N.W. Cant, E.M. Kennedy and P.F. Nelson, J. Phys. Chem., 97 (1993) 1445. J.A. Rots, S.J. Korf, R.H.J. Veehof, J.G. van Ommen and J.R.H. Ross, Appl. Catal., 52 (1989) 131. N.W. Cant, C.A. Lukey and P.F. Nelson, J. Catal., 124 (1990) 336. J.H. Edwards, R.J. Tyler and S.D. White, Energy and Fuels, 4 (1990) 85.
Natural Gas Conversion IV
Studies in Surface Science and Catalysis, Vol. 107 9 1997 Elsevier Science B.V. All rights reserved.
Measurement of kinetic isotope effects over methane coupling catalysts in the presence of carbon dioxide. Noel W. Cant a, Peter F. Nelson b and Bronwyn L. D u f f y b School of Chemistry, Macquarie University, NSW 2109, AUSTRALIA CSIRO Division of Coal and Energy Technology, PO Box 136, North Ryde, NSW 2113, AUSTRALIA 1. I N T R O D U C T I O N Despite a decade of study, details of the mechanism of the catalytic oxidation of methane to C2 hydrocarbons remain uncertain. The original proposal of lto et al. (1) for Li/MgO catalysts was as follows [Li+O -] + CH 4 ----> [Li§
-] + CH 3
C H 3 + C H 3 ----ff C2H 6
2[Li§ -] ---> [Li§ -] + Li§ + H20 [Li+O2l + Li+[] + 89 ----> 2[Li+O-I
[ll [21 [31 [41
with step [1] fast and [3] or [4] rate limiting (where [] is a surface vacancy). However this is inconsistent with the observation that CD4 reacts considerably slower than CH4 (2,3). This deuterium kinetic isotope effect (KIE = rate(CH4)/rate (CD4)), which has also been demonstrated for other catalysts (4), implies that CH bond breaking, and hence step [ll, is the rate determining step. Experiments comparing the rate in the presence of H20 or D20 suggested that step [3] is unlikely to be rate limiting (2). Two recent reinvestigations of the phenomena with Li/MgO catalysts have reached somewhat different conclusions (5,6). Shi et al. (5), in experiments with the methane concentration held at 25%, reported that the KIE value dropped when the oxygen content was reduced from 25% to 2.5%. Modelling led to rate parameters which were consistent with the kinetic orders observed by Roos et al. (7) in experiments with CO2 in large excess, and also with the observed variation in KIE. However Cant, Kennedy and Nelson (6) found that this data set was inconsistent with both kinetic observations made in the absence of added CO2 and the constancy of the KIE in experiments with varying methane concentrations. These findings, coupled with the extent of isotope mixing in experiments with ~6Oj~802 (8), led to an alternative data set which reaffirmed [1] as the limiting reaction. One possible explanation for these discrepancies is that at the lowest oxygen concentrations used by Shi et al. (5), inhibition by CO2 brings the rates of steps Ill and [4] closer together and this affects the measured KIE. The present study was designed to test this possibility through a determination of both the KIE, and the ~602/~gO2 mixing rate, in the presence of carbon dioxide. Some of the experiments used ~3CO2 in order to allow the extent of total oxidation of methane to be assessed even though the added carbon dioxide is much in excess of that produced by reaction. 2. E X P E R I M E N T A L The experimental procedures were similar to those described previously (3,6,8). The catalyst samples (200mg) were packed in a 4mm ID fused alumina tube between upper and lower thermocouple wells of 3mm OD and heated in a tube furnace. The standard reaction mixture was supplied by three mass flow controllers (Brooks Inst.) delivering CH4 (Matheson UHP grade), an analysed 12.5%O2/He mixture and diluent helium (CIG UHP grade). Additional mass flow controllers provided CD4, a
356
10%1sO2/He mixture and either ~2CO2 o r 13CO2. The CD4(>99%D) and the ~sO2 (>98%180) were obtained from CIL. The CD 4, unlike that used previously (2,3,6), was sufficiently pure that no correction of the product analyses for the presence of higher hydrocarbons was necessary. The ~3CO2, obtained from MSD Isotopes, was of high 13C purity (>99.3%) but it did contain, as noted later, some ~3cl6olSo. The product mixture from the reactor, or the reactants on bypass, were sampled by a two column gas chromatograph and a quadrupole mass spectrometer (VG model SX200) and then passed through a 10cm gas cell. The contents of this were analysed off-line by a Digilab model FTS20/80 PTIR with 256 scans of resolution 0.25 cm ~ collected for each spectrum. The present experiments used a new batch of catalyst made by the procedure described by Edwards et al. (9). Its lithium content after an initial calcination for 8 hours at 900~ was 0.2 wt% and the surface area 0.2mE/g. All kinetic experiments were carded out with conversions of the limiting reactant (02) held below 30% to allow calculation of rates using the differential reactor approximation. 3.
R E S U L T S AND DISCUSSION
Fig. 1 shows the effect of added 13CO2 on the rate of methane oxidation to each product in experiments using 50%CHJ5%O2 at 728~ Formation of all products except carbon monoxide are reduced to a significant degree. However, the EC2 selectivity remains constant at = 80%. The apparent reaction orders in carbon dioxide as calculated from the slopes of log-log plots are shown in Table 1. Formation of ethane and CO2 are inhibited to similar extents with apparent orders of-0.47 and -0.54 respectively. Production of ethene and C3 compounds are affected more (orders of-0.70 and -0.95) as expected since they are secondary and tertiary products respectively. The effect on carbon monoxide production is much less (order-0.1) probably because a reduced rate of formation from methane is partially compensated by additional production by the reverse water gas shift reaction between co-product hydrogen and added 13CO2 C O / + H 2 ---) CO + H20
[5]
to produce ~3CO as demonstrated by the FTIR spectrum in Fig. 2B. However this reaction is far from equilibrium. The 13C/~2C ratio in the carbon monoxide (= 0.8 from Fig. 2B), is very different to that in
,n. o
10
I
I
9 \
x
~"
~X
8 -
s
_
~
~ \
E
Table 1 Apparent reaction orders in CO2
1
9
CO
9
CO 2
9 A 9
Ethene Ethane C3's
-
Product C2H 6 COz C2H 4 Y'.C3 CO
B
'10 t~ L-
o G)
0 0
1
2
3
4
Average 002 Concentration (% v/v) Figure 1 728~
Effect of ~3CO2 on product formation at
order -0.45 a, _0.54 a -0.70' -0.95" -0.09 a
-0.41 b _c -0.69 b -0.69 b 0.07 b
' At 728~ with 50%CH4/5%Oz at 30 cm3/min, 200 m g catalyst. b At 778~ with 20%CH4/10%O2 at 40cm3/min, 200 mg catalyst. c ~3CO~ not used.
357
1.6
12002 ~ 13CO2
1.2
A
.8 .4 m 0.0 o r=
720
700
680
660
640
620
600
.04 .03 .02 .01 0.00
I
I
I
I
I
2200
2150
21 O0
2050
2000
Wavenumbers
(r
"1)
F i g u r e 2 FTIR spectra of CO and CO2 from reaction of 50%CH4]5%O2/2.5%13CO2 at 728~ the carbon dioxide (= 14 from Fig. 2A and product analyses). Fig. 2A also shows an additional Q branch at 645cm 1, attributable to 13C160180, which was found to comprise =7% of the starting 13CO2. Experiments to determine the effect of carbon dioxide on the 1602/1802 mixing reaction 1602 "t" 1802 It~ 2160180
[6]
were therefore carried out with 12CO2 rather than 13CO2 added as the 180 in the latter would have complicated the analyses unduly. Fig. 3A shows the 1602, 160180, 1802 distribution with no added CO2 while Fig. 3B shows the effect of CO 2 addition on the extent of mixing (the fractional approach to equilibrium), X m, defined as X m = [(16O180)out - (160180)in]/[(160180)** - (160180)in ]
[71
It is apparent that the exchange reaction is inhibited to a much greater extent than was production of ethane and carbon dioxide (Fig. 1). X m has a value of 0.30 in the absence of added CO2 but this falls to 0.017 when the average CO2 concentration reaches 4.75%. A log-log plot of the data gave an apparent order of -1.1 in CO2, double that for the inhibition of ethane production in Table 1. It may be noted that X m reached 0.94 in an experiment using 1602/1802/He alone (ie without methane) under the same conditions. Thus the CO2 produced by coupling alone is able to drastically obstruct the oxygen mixing reaction. (The reduction in Xm from 0.94 to 0.30 is equivalent to reduction in rate by a factor of almost l0 assuming that the isotope reaction is first order in the distance from equilibrium). Methane coupling and oxygen exchange may occur on different types of surface sites, however, the greater effect of CO2 on isotope mixing compared to methane coupling can be readily rationalised as follows. In terms of a single oxygen molecule, equation [4] is
358
2[Li+O 2] + 2Li+[] + 02 --~ 4[Li+O -]
[81
The forward reaction yields 2 O which may initiate methane coupling via hydrogen abstraction. However reversal of reaction [8] does not, on its own, produce isotope mixing. That requires dissociation of 1602 and 1802 molecules on adjoining sites and their random recombination. Thus the site requirement for mixing is larger than that for methane activation. If carbon dioxide blocks a large fraction of the surface, as seems likely, then some of the remaining sites may still be able to achieve oxygen dissociation, and then methane oxidation, but not oxygen isotope mixing. As a further consequence it is not possible to accurately estimate the true rate of oxygen recombination (the reverse of step [4]) from the amount of 160~80 evolved when much CO2 is present, but only a lower limit to it. Additional amounts of 1602 and 1802 may dissociate on isolated sites where recombination cannot give rise to isotope interchange. Although the oxygen isotope mixing reaction, [6], is far from equilibrium (Fig. 3A) and still further from it with added CO2 present (Fig. 3B) the three isotopic carbon dioxides in the exit gas are fully equilibrated as illustrated by the FTIR spectra of Fig. 4A. If the CO2 produced by reaction was simply additional to that introduced as 12CO2, then the distribution between C1602 , C160180 and C~802 would have been 0.80:0.13:0.07. However, the observed distribution was 0.75:0.23:0.02 which agrees with the equilibrium one within the experimental error as shown in Fig. 4B. Table 2 summarises the results of experiments to determine the deuterium kinetic isotope effect (KIE) in the presence and absence of added CO2 under two sets of conditions. The pattern of KIE values, and the overall effect (1.5 to 1.6) is similar to that reported previously (2,3,6). Shi et al. (5), in experiments with no added CO2, reported lower values when using high CHJO2 feed ratios. This is not apparent in the present data even though the 5 0 % C H J 5 % O 2 mixture used in the experiment at 728~ has the same ratio as the highest one used in their work (25%CHJ2.5%O2). As may be seen from Table 2, the presence of 1.5%13CO2 at 769~ with a CHJO2 ratio of two, has little effect on the distribution
359
of KIE values. It is possible that a small reduction in the KIE to C2H 6 and EC2 occurs when 2.5%13CO2 is added to the 50%CH4/5%O 2 feed at 728~ but the accuracy of these measurements is somewhat less due to low conversions. Unfortunately the determination of 12CO2by FTIR under these conditions was too inaccurate to provide a reliable estimate for the KIE to this product and therefore for the overall reaction. Overall it appears that the rate determining effect retains a large component of carbon-hydrogen bond breakage when a large excess of CO 2 is present even though this interferes with oxygen dissociation/recombination, as revealed by oxygen isotope mixing (Fig. 3B) and induces a different kinetic regime (7). While no exact determinations of the kinetic orders in CH 4 and 02 in the presence of CO 2 were attempted here, two-point estimates indicated that they approached those expected from the data of Roos et al. (7), first order in O 2 and zero order in CH4, and were considerably different from
Table 2 KIE values for methane coupling with and without added '3CO2
% 13C02
KIE = rate(CH4)/rate(CD4)
Conditions
added
C2H 6
C2H a
CO 2
CO
EC 2
overall
20% methane, 1 0 % 02 769~ 50cm3/min.
nil 1.5
1.48 1.54
2.2 2.7
1.25
1.47
1.1 1.3
1.66 1.65
1.58
50% methane, 5% 02 728~ 30cm3/min.
nil 2.5
1.50 1.38
2.4 2.6
1.17 -
1.0 1.2
1.61 1.45
1.51
1.51 -
360
those found previously (6) for reaction in the absence of added Table 3 CO2 (0.3 and 0.7 respectively). Oxygen atom recombination relative to methane On the basis of measurements consumption using 50%CH4/2.3%1602]2.3%1802 at with 90%CH4/5%1602/5%lsO2 728~ mixtures (8), and modelling for other conditions (6), we have previously concluded that the % CO 2 Xm ECH4 O....>O2]CH4 a number of oxygen atoms which added (loss,%) (loss) undergo dissociation and recombination to oxygen molecules, nil 0.46 3.1 2.2 ie step [4] and its reverse, is 2 to 20 times as great as those which attack 2.2 0.045 1.0 0.8 methane. Table 3 repeats this calculation for experiments using 50%CH4/2.3%I602/2.3%lsO 2 mixtures and carded out as part of " equals 4Xm[O2]~CH4(loss ) Ref.(6) the sequence to determine the KIE at 728~ In the absence of added CO2 the rate ratio falls at the lower end of the above range as expected for conditions when the methane concentration is high and the oxygen concentration low (6). With 2.2% CO2 present the calculated rate ratio does falls below unity because 160180 is inhibited more than methane. However, as explained in connection with the data of Fig. 3 previously, this is only a lower limit since the 1601sO formation rate does not provide a good estimate for the rate of the reverse of step [4] when CO2 occupies a large fraction of the surface. 4. CONCLUSIONS (1) The present results indicate that the oxygen species which attacks methane in the coupling reaction is the same as that involved in the oxygen isotope mixing reaction. The greater effect of carbon dioxide on the latter reaction can be explained in terms of its larger site requirement. (2) A substantial deuterium kinetic isotope effect occurs in the presence and absence of CO 2 indicating that CH bond breaking is rate limiting in both situations. There may be a slight reduction in the KIE when using both a high CHJ02 ratio and a large excess of CO 2 but the data is insufficiently accurate to establish this with certainty. (3) Finally while the present results are readily interpreted in terms of the model described by steps [1] to [4], which is based on the existence of surface O species, this should not be taken as proof of it since similar models based around other oxygen species are possible. 5. R E F E R E N C E S 1.
T. Ito, J.-X. Wang, C.-H. Lin and J.H. Lunsford, J. Am. Chem. Soc., 107 (1985) 5062.
2. 3. 4. 5. 6. 7. 8. 9.
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