J. Phys. Chem. 1994, 98, 13357-13372
13357
Radical Chemistry in Methane Oxidative Coupling: Tracing of Ethylene Secondary Reactions with Computer Models and Isotopes C. A. Mims* and R. Mauti Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, MSS 1A4 Ontario, Canada
A. M. Dean and K. D. Rose Exxon Research and Engineering Company, Corporate Research Laboratories, Route 22E, Annandale, New Jersey 08801 Received: June 16, 1994; In Final Form: September 18, I994@
The reaction pathways of ethylene during methane oxidative coupling over (Li)MgO catalysts were investigated by detailed isotopic analysis of all the products from 12C2&: 13CH4:02reactant mixtures. Ethylene oxidation has little effect on the concurrent methane conversion, even at comparable mole fractions. By comparison with an extensive computer model of the gas-phase chemistry, the surface is seen to effectively quench much of the oxidative gas-phase radical chemistry. The detailed C3 and C4 hydrocarbon yields and isotopic distributions, however, are well described by gas-phase hydrocarbon radical pathways.
Introduction In the oxidative coupling of methane, as in all selective oxidation reactions, secondary reactions of the desired product, ethylene in this case, are of central importance. Most significantly, the deep oxidation of ethylene limits the ultimate yield of higher hydrocarbon products. An empirical upper limit to the higher hydrocarbon yield exists which results from decreasing higher hydrocarbon selectivity with increasing A large world-wide research effort has not resulted in significant improvement. Several novel process concepts7-’ are being pursued in an attempt to bypass this limit. In addition to oxidation, ethylene also participates in secondary reactions which lead to molecular weight growth. These reactions are of some economic importance in the reaction, open possibilities for coprocessing, and also provide information about the reaction mechanism. Increased understanding of the reaction of ethylene during methane coupling provides guidance both in the continuing search for improved catalysts and in the investigation of novel process configurations. Oxidation of ethylene is only one possible cause of the increase in COXproduction at high conversions. The complexity of the reaction network, which comprises both an extensive gasphase radical network and surface processes which are not well understood, provides the possibility of strong coupling among various parts of the reaction mechanism. The critical issues in the formation of deep oxidation products are (1) the surface and gas-phase processes (and their relative importance) which govem the initial C2+ selectivity and similarly ( 2 ) the processes which govem the further reactions of ethylene, particularly oxidation. Much detailed understanding has been obtained from investigations of the reaction mechanism, including the source of the deep oxidation products. Isotope tracer experiments have been used to unravel some of these issues,11-16 and it has been shown12 that, on (Li)MgO at low ethylene concentrations, the initial methane selectivity toward ethane is unaffected by ethylene addition, implying that little feedback exists from the ethylene conversion reactions and methane coupling. Tracer studies of (Sr)LazOs at higher ethylene concentrations gave @Abstractpublished in Advance ACS Abstracts, November 1, 1994.
similar re~u1ts.l~Ekstrom et al.14 showed that with Sm203 there is cross talk between the rates of methane conversion and conversion of C2 hydrocarbons. The use of computer models of the gas-phase radical chemistry has provided insight into the source of some of these i n t e r a c t i o n ~ . l ~Homogeneous -~~ gas-phase models have shown that methyl oxidation is increased dramatically by an increase in undesirable reactions (especially those involving HO2) that arise from ethane conversion chemistry.” Important peroxide chemistry is easily interrupted by some solid surfaces, and such surface interference effectively quenches many of the gas-phase radical oxidation^.^^-^^ The higher hydrocarbon growth pathways in methane coupling have received less attention. ARC0 workers noted that the distribution of higher hydrocarbon products from methane coupling was similar to that from homogeneous methane oxidation and methane pyrolysis.27 Addition of olefins to the methane-oxygen reaction significantly increases the production of higher hydrocarbon^.^^-^^ Simple reaction networks have been used to simulate these processes, but a detailed examination has not been conducted. This paper describes the results of a detailed investigation of ethylene secondary reactions during methane oxidative coupling catalyzed by lithiated magnesium oxide. We use detailed isotopic analysis of the products of the oxidation of 13CHq12C2& mixtures together with computer simulations to aid in the interpretation of the results. The primary focus of this paper is the study of the higher hydrocarbon formation pathways and the extent to which gas-phase radical chemistry can explain the detailed results. The model we use in the simulations contains an extensive set of gas-phase radical reactions (through C4 species). Selected surface reaction steps are added to initiate and alter the gas-phase chemistry. In addition to the higher hydrocarbon formation reactions, we also present the details of the deep oxidation processes and compare them to previous studies. Although surface reactions are clearly involved in the deep oxidation steps, we do not try here to simulate these in detail in an attempt to fit the data exactly. The suite of surface reactions and their rate parameters are still very uncertain and catalyst specific. Rather, the aim here is to provide a simulation
0022-3654/94/2098-13357$04.5~IQ 0 1994 American Chemical Society
Mims et al.
13358 J. Phys. Chem., Vol. 98,No. 50, 1994
f c-c-c
a
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nism has been used previously to model methane pyrolysis.32 Although some of the rate constant parameters in the current model differ from this earlier study, no significant changes in the model predictions result from these changes. Surface reactions are included as desired and treated as additional homogeneous reactions. Two versions of the model are used, which differ in the “surface” reactions included. In the first (I), the only “heterogeneous” step included is the activation of methane, CH,
Figure 1. Schematic representation of significant gas-phase reaction pathways during methane oxidative coupling. Solid arrows represent CHs additions; dotted arrows indicate H atom loss (abstractors not shown); dashed arrows are oxidation steps. Reaction partners are indicated for clarity where needed.
of the behavior expected in limiting cases with minimal surface involvement. These, by comparison with the experimental results, provide indirect information about the degree of surface involvement.
Reaction Paths and Computer Model Figure 1 shows, in simplified schematic form, the reaction network used in the computer model. The computer model is described more fully in the Appendix where a listing of the reactions and the rate constant parameters can also be found. It is a homogeneous, one-dimensional (time) model (thus simulating perfect plug flow) and contains nearly 500 elementary gasphase reactions, a large portion of which are necessary to describe the C3+ chemistry. The rate constant parameters were carefully chosen from published data and were not optimized around our catalytic results. The reaction set, exclusive of the C3+ species, is similar to previously published models. However, a significant feature of the model is that many reactions contain contributions from thermally nonequilibrated (or chemically activated) reaction complexes and were treated in the model by the QRRK procedure previously describede31 Furthermore, the reaction set involving the C3+ species is the first to examine these processes with such detail. Within each carbon number, dehydrogenation of stable molecules involves H-abstraction by OH, H, and CH3 (and by 0 2 for weak C-H bonds). The radicals can lose H by simple elimination or by loss to 0 2 to form H02. Oxidation channels for each radical consist of reactions with HO2 (and other peroxy radicals) as well as direct reactions with oxygen. The reactions of C, alkyl radicals with HOz were assumed to produce OH along with CH2O and C,-1 radical (from the thermolysis of C, alkoxy radical). Vinylic radicals react very rapidly with 0 2 . Oxidative attack of the stable species (especially alkenes) by 0 and HO2 were calculated to be unimportant under OUT conditions. Carbon number growth involves addition of radicals to unsaturates (followed by H-elimination) as well as radical-radical combinations. Conversely, larger radicals can degrade by ,&elimination. The model through the C2 stage has been validated against homogeneous methane partial oxidation data (see the Appendix). The nonoxidative part of the molecular weight growth mecha-
+ MO -CH, + MOH
(1)
to produce methyl radicals, amply shown to be the initiation step of the reaction. The initiation rate constant is adjusted so that the methane conversion in the simulation matches the experimental value when the simulation tim is equal to the experimental gas residence time in the catalyst bed. In the second version of the model (11), one additional heterogeneous step is included which destroys HOz radicals,
H02
+ MO -,MOH + 0,
(2)
The value of this rate constant is set as high as could be justified on the basis of surface collision rates, which effectively quenches many of the HO2 reactions with hydrocarbon radicals. Since the reaction of HO2 with CH3 is the dominant C1 oxidation pathway in version I, the oxidation rates are strongly quenched in version II. Furthermore, the OH radicals produced in the HO2 reactions are the dominant H atom abstractor in the fully developed radical mechanism. This chemistry is effectively quenched in version 11, which therefore simulates the minimum amount of dehydrogenation chemistry. These two versions of the model simulate two extremes in the degree to which surface reactions interfere with the oxidative gas-phase chemistry. In both cases, the site regeneration reactions 2MOH
+
H20
+ M 4- MO
(3)
and
0,
+ 2M
--+
2M0
(4)
are assumed to be rapid. No mechanistic conclusions are contained in this assumption, however. Thus both models simulate initiated gas-phase oxidation, one fully developed and the other moderated by surface influences. Analysis of the rates of the carbon flow through the individual reactions during the simulation allows the calculation of an expected isotopic distribution in the products of a l3CI-I412C2H4-0z experiment. While detailed tracing could be done by brute force, this would require 2, isotopomers for each C, species (over lo00 total) and over 10 O00 reactions in the model. We (and the computer) deemed this impractical. Instead, we calculate the isotope distributions by following the flow of carbon from ethylene (initially 12C)and methane (initially I3C) through the reaction manifold in sequence and applying a rough integration of these rates. Some recycle loops exist in the mechanism which require care during this procedure, and these will be discussed with the results.
Experimental Section A lithium-magnesia catalyst was prepared for these experiments according to the method described by Lunsford et al.33-35 A slurry made from thoroughly hydrated magnesia and a solution of Li2CO3 was dried, calcined, pelletized, ground, and sieved to produce particles with diameters of 80-120 pm. The
J. Phys. Chem., Vol. 98, No. 50,1994 13359
Radical Chemistry in Methane Oxidative Coupling catalyst had a final Li:Mg ratio of 0.29. The reactions were canied out in a tubular quartz reactor with a 4-mm inside diameter. The catalyst temperature was monitored by a thermocouple in a 1-mm quartz sheath embedded in the catalyst charge. The reactor narrowed to 2-mm i.d. below the catalyst bed to minimize gas-phase reactions in the post-bed region. Typically 0.25 g of catalyst rested on a shallow platform of quartz wool. The reactant gas mixtures were synthesized by mass flow controllers. The reactant gases were obtained from Matheson at stated purities of 99.99% and used without further purification. The I3CI-I4 was obtained from ICON and was analyzed to have 99.6% 13C. Steady-state reaction rates were measured by GC analysis of the effluent for reactions of the single components (methane and ethylene) and for mixtures of methane and ethylene. For the 13CH4-C2H4 tracer experiments, a steady-state reaction is first established and the rates and selectivities are measured. Then 13CH4 is substituted for the methane, and the products are analyzed for their detailed isotopic composition in the manner described p r e v i o u ~ l y .The ~ ~ light gases (including the C2 hydrocarbons) were analyzed by GCMS. The C3+ material was trapped in a liquid-nitrogen-cooled loop during the experiment and subsequently separated by GC (while performing GCMS) into carbon number fractions for proton and 13CNMR analysis. Approximately 2 pmol of labeled C3 and 0.5 pmol of C4 products were collected. From these measurements, a complete breakdown of the products into their isotopic variants is obtained. The proper determination of the isotopic label in the CO, products requires care. Carbon dioxide exchanges rapidly with the large carbonate phase37-39on this catalyst, and the establishment of a new isotopic equilibrium can consume much time and money. The achievement of isotopic steady state is also difficult to verify since the approach to steady state is gradual when diffusive mixing into a bulk phase is important. We achieve isotopic balance in a much shorter time by adjusting the methane isotopic composition to achieve a CO, product which matches the isotopic composition of a prelabeled catalyst. Figure 2 illustrates this procedure for the results described below. In panel a, the rates of change in the CO, 13C content is monitored in the reaction product of a CH4-02-Ar mixture. In each panel, methane with a different 13C content is used. The rate of change of 13C in the product is a measure of the sign and degree of mismatch between the isotopic composition of the nascent COXand the catalyst. These results are plotted in panel b, and the balance point can be obtained by interpolation. The data in Figure 2 indicate that 55% 13C-labeled C& yields COz which is in isotopic balance with the catalyst, which contains approximately 10% 13C.
Results and Discussion Catalyst Performance and Simulations: C h - 0 2 . Figure 3a shows the results of a residence time study showing the higher hydrocarbon product selectivities as a function of methane conversion at standard conditions (CH4:02:Ar = 2: 1: 4; T = 700 "C). The results are in general agreement with previous reports. Finite selectivities at low conversion establish ethane and C02 as primary stable products. Hutchings et al. reported very high CH2O selectivities at very low conversionsa which we do not reproduce. Also in agreement with other studies?l the initial selectivity to C02 generally increases as the catalyst ages, and variations among the CO, selectivities in the various data sets in this paper result from catalyst aging. Dehydrogenation of ethane produces ethylene, which as a secondary product is formed with zero initial selectivity. The
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0.10
6" 0
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0.09
0
200
400
600
800
1000
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P
-60-
'
1
5
Figure 2. Tracing of the source of carbon dioxide by altering methane isotopic composition to match catalyst carbonate phase. Panel a: I3C content of Cot product as a function of time. Methane with different amounts of 13Cis used in each of the five time periods. Panel b: Rate of change in 13C fraction in CO2 as a function of 13Ccontent in CH4. Numbers by data points correspond to time periods in panel a.
initial selectivities to C3 and C4 hydrocarbons are also zero and increase only after appreciable amounts of ethylene are formed. At higher conversions, the selectivities to CO, also increase. These general features are produced by the gas-phase model in panels b and c of Figure 3. Since these calculated C1-C2 selectivities are similar to those in previously reported studies, we only discuss them briefly. The predicted hydrocarbon products are in general agreement with the experimental results. Model I predicts the appropriate amount of ethylene, while the decreased dehydrogenation in model I1 results in a lower ethy1ene:ethane ratio. The amounts of C3 and C4 hydrocarbons predicted by both models are in reasonable agreement with the experimental results. The quenching of the radical chemistry in version I1 results in a general decrease in dehydrogenation processes so that the detailed product distributions predicted by the two models differ significantly (see below) and are apparent in the methane-ethylene cofeed results below. The initial selectivity to oxygenated C1 products is low in both simulations. The predominance of CO and CH20 seen in homogeneous methane oxidation s t u d i e ~ is ~ ~reproduced -~ by the model and contrasts with the predominance of CO2 in the catalytic data. The two models predict very different dependences of the C1 oxide selectivities with conversion, owing to their very different treatment of HO2. Model I predicts a more rapid increase than that seen experimentally, while model I1 predicts a much smaller change. Catalytic Results: C&-Cz-02. Figure 4 shows the resulting changes in the product distribution at a single gas residence time as increasing amounts of ethylene are added to the methane-oxygen mixture. The C3+ hydrocarbons and COX all increase in a roughly linear manner as ethylene is added. The ethane formation rate decreases slightly with added ethylene. Figure 4 also shows the predictions of both versions of the computer model. They both predict a similar decline in the
13360 J. Phys. Chem., Vol. 98, No. 50, 1994
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Methane conversion Figure 3. Dependence of reaction selectivities (carbon atom basis) on methane conversion studied by residence time variations over 0.27 g of 5 wt % Li on MgO. Reaction conditions: 975 K, 105 kPa total pressure. Inlet mole fractions: 0 2 = 0.125; CHq = 0.25; balance Ar. Panel a shows experimentalresults; panel b, results simulated by model I; and panel c, results simulated by model II. (0)= C2&; (0)= CzHq; (A)= C3; (m) = Cq; legend for lines is consistent throughout. production of ethane with increasing ethylene concentration which is somewhat more pronounced than that seen experimentally. The C3+ hydrocarbons increase almost linearly with ethylene concentration and in semiquantitative agreement with the experimental data. Because of the different treatment of HOz in the two models, they predict widely divergent COX responses. Version I of the model predicts a stronger increase in COXthan is seen experimentally, while version I1 predicts little increase. A more detailed analysis of the product distribution is performed with the tracer experiment. Catalytic Results: Cfi-02. The reactivity of ethylene was also measured for comparison with the results of the ethylenemethane mixture. The residence times and temperatures of this study are in the threshold region for “ignition” of homogeneous ethylene-oxygen a fact which produces complex rate behavior. At ethylene (0.10 atm) and oxygen (0.15 atm) pressures and flow rates typical of this study, an empty reactor provides sufficient residence time for autoignition to occur, accompanied by complete oxygen utilization. In such circumstances, the presence of (Li)MgO actually slows the reaction and postpones the ignition to higher temperatures, ethylene pressures, or conversion^.^.^^ At conditions which are insufficient to reach the autoignition point, the catalyst causes an increase in the ethylene conversion rate. The presence of methane also reduces the tendency toward autoignition. The tracer experiments reported here were performed at conditions below the autoignition threshold. The products from ethylene oxidation are predominantly C1 oxides in all cases, with minor amounts of methane and C2 oxygenates and only traces of C3+
Ethylene mole fraction Figure 4. Dependence of reaction rates (carbon atom basis) on inlet ethylene mole fraction over 5 wt % Li on MgO. Reaction conditions: 975 K, 105 kPa total pressure. Inlet mole fractions: 0 2 = 0.125; CHq = 0.25; balance Ar. (m) = 0.27 g of catalyst, (0)over 0.23 g during COz tracer described in Figure 2 and Table 1; (A)0.37 g obtained during the higher hydrocarbon trap experiment described in Table 2. Solid and dashed lines are the predictions of models I and 11, respectively. hydrocarbons. The results are similar to those obtained by Sanchez-Marcana and co-workers.46 C2J&-13CH4-02 Tracer Experiments. Several tracing experiments were performed at various C2H4 partial pressures. Many aspects of the isotope distribution obtained by routine GCMS were measured in all these experiments. However, the COz isotopic distribution was only measured twice because of the associated expense, and the full N M R determination of the isotopomer distribution was performed only once. That the conclusions from these special experiments are generally applicable to a wide range of conditions is assured by agreement with the information obtained by routine GCMS analysis. Overall Carbon Flow. A summary of the total flow of carbon from both methane and ethylene into the various products was obtained from the experiments where the COX isotopic compositions were measured. These results are given in Table 1 along with the reaction rates of the components when reacted
Radical Chemistry in Methane Oxidative Coupling
J. Phys. Chem., Vol. 98, No. 50, 1994 13361
TABLE 1: Rates @mol of C/s(mL of Void Volume in the Catalyst Bed)lb cH4-02
XO(CH4) XO(C2H4) CH4 feed rate C2H4 feed rate CHq rates cH4 to co cH4 to c02 CHq to CH20 cH4 to cox cH4 to
C2H6
CHq to c2H4 cH4 to c3 CHq to c.4 cH4 to HHC total CHq rate selectivity to C2+
C2H4 rates c 2 H 4 to co c 2 H 4 to c02 C2H4 to CH2O c 2 H 4 to cox C2H6 c2H4 to CHq C2Hq to C2 oxygenates c 2 H 4 to c 3 c 2 H 4 to c 4 c2H4 to HHC c 2 H 4 to
total C2H4 rate selectivity to C3+
0.25 0.00 50.5
0.13 0.99 0.02 1.14
1.31 0.20 0.02
