Faraday Discuss. Chem. SOC., 1989, 87, 47-64 GENERAL DISCUSSION Prof. M. S. Spencer (University of Wales, Cardifl) (communicated). Much of my comment applies generally to work on methane coupling rather than specifically to Prof. Lunsford’s paper. For more than half a century gas-phase oxidation has been intensively studied, including the selective oxidation of methane,’ but little of this work is considered in the interpretation of methane-coupling results. Heterogeneous reaction steps are commonplace in nominally ‘homogeneous’ systems. Diffusion paths in a methane- coupling catalyst are typically a factor of lo6 shorter than in a reactor used for gas kinetics, so heterogeneous steps must be expected in some, if not all, of chain initiation, propagation, branching and termination.It is folly to attempt an arbitrary separation of the process into heterogeneous and homogeneous reactions. Professor Lunsford’s experiments to measure directly the efficiency of catalyst sur- faces in methyl radical generation and reaction are most welcome. It would be interesting to see these results, and others, used in an analysis of methane coupling as a conventional ‘homogeneous’ methane-oxygen chain reaction, with added heterogeneous steps for, e.g. chain initiation ( CH4 dissociation), branching [CH,02(a) ---* ?] and termination [2CH,(a) -+ C2H61. 1 D. M. Newitt and P. Szego, Proc. R. SOC. London, Ser. A, 1934, 147, 555. Prof. J. H. Lunsford (Texas A&M University, Texas, U S A . ) replied: The modelling of coupled heterogeneous-homogeneous reactions which occur during CH4 oxidation is, of course, an important objective for researchers in the field.Such models are presently limited by the almost complete absence of rate data on the reactions that occur between the radicals and the metal oxides that are commonly employed. For example, the initial reaction between a CH; radical and a metal oxide is a matter of conjecture, and the absolute rates of such reactions are not known. Clearly, additional research is needed in this area of kinetics and surface dynamics. Prof. M. W. Roberts (University of Wales, Cardifl) said: You mention the possibility of one of three different oxygen species being active in CH4 activation: 0-, 0, and 06-. We have drawn attention elsehwere to experimental evidence for the high reactivity of transient surface oxygen species in H-abstraction reactions.”2 The stable chemisorbed oxygen species are unreactive under the same experimental conditions. Do you think that the active species may be surface oxygen transients, possibly vibrationally excited? 1 A.F. Carley, M. W. Roberts and S. Yan, J. Chem. SOC., Chem. Commun., 1988, 267. 2 C. T. Au and M. W. Roberts, J. Chem. SOC., Faraday Trans. 1, 1987, 83, 2047. Prof. J. Cunningham (University College Cork) followed on from this: I would like to address Prof. Robert’s question as to the possible similarity between Lif/NiO and Li’/MgO, and develop an ensuing clarification that the positive hole in Li+/MgO is located on oxygen, whereas that in Li’/NiO is located on nickel. This ‘positive-hole’ character of 0- in MgO is represented within the Kroger-Vink notation by assigning a positive charge to that region of an MgO lattice which contains an 0- at a site normally occupied by 02-.As a consequence of this overall positive charge, such a ‘lattice’ 0- species can indeed be expected to exhibit properties strongly modified from those reported for 0- species in the gas phase. On this basis I do not have any difficulty with Prof. Lunsford’s adoption of a different notation namely 0’- in his paper, to distinguish it from 0-(g). Carried through to its logical conclusion, it suggests that a species such 4748 General Discussion as 0; formed from reaction of a lattice 0- with gaseous O2 would have significantly different properties from gas-phase 0,. Prof.Lunsford responded: In a sense we view the ozonide ion, O,, as a transient oxygen species which is vibrationally excited. Previous e.s.r. results have shown that 0; ions on MgO are unstable over a period of several hours at room temperature, and under reaction conditions the surface reactions 0-+o, s 0; 3 o;+o may 0ccur.l With our quenched Li+/MgO samples 0, has also been detected.* The availability of electrons for reduction of oxygen, however, is a major difference between Li+/MgO and the Zn (0001) surface which you have examined. In the former case the number of such electrons is very limited, whereas in the latter case they are available to form transient species such as surface O,, which is ultimately reduced to surface and bulk 02- ions. 1 N. B. Wong and J.H. Lunsford, J. Chem. Phys., 1972, 56, 2664. 2 J-X. Wang and J. H. Lunsford, J. Phys. Chem., 1986, 90, 5883. Dr R. Burch (Reading University) continued: The question I should like to raise concerns the actual coupling step in the formation of ethane from methane. It is clear from your earlier work that methyl radicals are released into the gas phase when methane is passed over Li/MgO catalysts in the presence of oxygen. However, Hatano and Otsuka' have recently claimed on the basis of kinetic measurements that methyl coupling occurs on the surface of an Li/NiO catalyst. Similarly, Wada et ~ 1 . ~ have argued on the basis of flow-rate studies that no ethane is formed in the gas phase over La/B203 catalysts. Do you have any information on the rate of release of methyl radicals from oxides, such as Li/NiO, in which one of the cations has clear redox characteristics? Is there any reason to exclude the possibility of surface coupling over catalysts of this type? 1 H.Hatano and K. Otsuka, J. Chem. SOC., Faraday Trans. I . , 1989, 85, 199. 2 S. Wada, T. Tagawa and H. Imai, Appl. Catal., 1989, 47, 277. Prof. Lunsford replied: Preliminary experiments on an Li+/ NiO catalyst using our MIESR system revealed that the rate of formation of gas-phase CH; radicals was unexpectedly low relative to the overall catalytic properties. It may well be that there is a class of catalysts which binds CH; radicals to the surface in such a way that coupling can occur prior to more extensive oxidation of the reactive intermediate. Prof. Cunningham said: A simple calculation indicates that, for an MgO powder of surface area ca.40 m2 g-I, the amount of Li+ sufficient to yield one monolayer coverage on MgO is only ca. 0.2 wt%. Consequently, in our studies at University College Cork upon Li'/MgO and other surface-doped MgO materials we have focused our main efforts upon materials with wt% loadings equivalent to monolayer coverage amounts, in the belief that effects of the dopants upon oxidative dimerization and other processes would thereby originate mainly from surface and near-surface regions. This belief has been strengthened by the results of depth-profiling studies upon Li+/ MgO materials, obtained in collaboration with Prof. Hirschwald at the Free University, Berlin, and now shown in fig. 1. This figure contrasts the relatively shallow depth-profile obtained by dynamic SIMS for lithium close to the surface of a monolayer-doped 0.24 wt% Li+/ MgO material, with the presence of lithium at greater concentration and to much greater depth in a Li+/MgO material prepared in identical fashion but with a lithium loading equivalent to ca.10 monolayers.General Discussion 49 1 . 5 0.5 I I I I 1 15 45 75 105 135 165 sputtering time/min Fig. 1. Depth profiles of Li/MgO at a fixed current density of 200 nA ern-:, but at different Li loadings: ( a ) 0.15 wt% of Li/MgO, and ( 6 ) 2.4wt% Li/MgO. Prof. Lunsford added: Your depth-profiling results raise some interesting questions concerning the influence of the near-surface region in the oxidative dimerization of methane.Our earlier study' showed that upon increasing the amount of Li+ from 0.2 to 1 wt%: ( a ) the surface area decreased from 37 to 8 m g-', ( b ) the specific activity for CH, conversion increased by a factor of 7 and ( c ) the C2 selectivity increased from 18 to 55%. Thus, the specific productivity of C2s greatly exceeded the fivefold increase in Li+ loading. The profiles of fig. 1 clearly show that the region near the surface both is richer in Li+ and extends further into the MgO with the sample containing 2.4 wt% Li'/ MgO than with the other sample. The more intensive and extensive Li+-rich near-surface region may be responsible for the greatly improved catalytic results that were observed when the Li+ content was increased. Obviously, CH, is activated only on the surface, but the concentration of active centres at the surface (e.g. 0'- or 0,) may be strongly influenced by the presence of [ Li+O-] centres in the near-surface region via hole transport, i.e.[L~+o-]+ 0: I-* [L~+o'-] + 0,. 1 T. Ito, J-X. Wang, C-H. Lin and J. H. Lunsford, J. Am. Chem. SOC., 1985, 107, 5062. Prof. Cunningham made a third comment: On the basis of the lithium depth-profile data just illustrated for Li+/ MgO material doped at the monolayer-equivalent level, we considered it probable that modifications of the defect structure of such material, such as the proposed creation of (Li-0-) pair defects, would be concentrated close to the surface. A temperature-programmed oxygen isotope exchange (TPOIEX) procedure was therefore developed in order to establish whether the 0- ccjmponent of such defects would exhibit earlier onset temperature for isotope exchange with gas-phase than the lattice 0,- anions at the surface of undoped MgO.Our results from application of this TPOIEX procedure to Li'/MgO are summarised in fig. 2, which represents them in terms of the atom fraction of oxygen-18 remaining in the gas phase (having initial composition of 0.5 mole fraction of "02 plus 0.5 mole fraction '"0,) over Li+/MgO or over MgO whilst temperature was ramped from 620 to 760 "C. It is clear from this figure that an isotope exchange process involving the exchange of l80 from the gas phase with 0 from the surface of the solid oxide onsets at a much earlier temperature and/or is 1650 General Discussion 0.6 0.5 0.4 00 C w .- - 2 0.: ct Y 5 (d 0.; 0.1 c I I 1 I 1 I I I 610 630 650 670 690 710 730 750 770 temperature/"C Fig.2. Comparison of atom fraction 18 plots for ( x ) MgO and (0) Li/MgO: both pretreated in oxygen at ca. 480°C for 1 h. more efficient over Li+/MgO than over MgO. In our poster at this meeting we show that this process must be of a form usually termed R , , viz. ~ 1 8 0 n - + 160180 160;,, + 1802(g, ( 1 ) ( g ) - Since there have been many proposals that R,-type oxygen exchange proceeds via a triatomic oxygen intermediate,' our results are consistent with Lunsford's proposal in his paper that 0, intermediates may be important in the initial step for activation of CH, on Li+/MgO surfaces. 1 K. Tanaka, .I. Phys. Chem., 1974, 78, 555. Prof. 0. V. Krylov (Academy of Sciences, Moscow) (communicated): In connection with the paper presented by Prof.Lunsford I should like to present our data on the same topic. We studied the processes of reduction and re-oxidation of some catalysts for the oxidative coupling of methane-Li20/Mg0, PbO/A1203, K20/A1203. It was shown that the reduction of the samples with H2 and CH, proceeds in the temperature range 400-700 "C. Among the gaseuos products, C2H6, C2H4, C02, H20 were found. At temperatures 400-500°C the rate of H20 formation becomes much lower than that for H2 (CH,) consumption. Under these conditions the heats of hydrogen adsorption QH2 can be directly measured from calorimetric data. The energies of 0-H bonds, Eo-H,~ were calculated by ( 1 ) H2 + W N S --* 2(OH)s using the values of QH2, in table 1 .Surface 0 - H groups can react with the consequent formation of oxygen vacancies and following re-oxidation via the traditional scheme of re-oxidation including the dehydroxylation stage: 2(OH), 3 2(0),+0,+H,O (2) o+;o, - (O)',. (3)General Discussion 51 Table 1. The parameters of interaction of H7 and CH4 with samples: QH2, E , - , , activation energies of H2 ( EL2) interaction and C 2 hydrocarbons formation ( E & ) ; rates of C 2 hydrocarbons ( Wcz) and COz ( Wco,) formation and rate of Hz consumption ( W,?) Li20/Mg0 210 95 90 320 7.5" 0" 31" K 2 0 / Alz03 1 10 52 120 270 l S h 4.3 71h PbO/A1203 55 80 180 250 1.7h 1.9h 154h "650 "C; h700 "C. But the interaction of oxygen with Li20/Mg0 pretreated in CH, or H2 leads to the formation of H20, i.e.the re-oxidation process proceeds as some sort of oxidative dehydrogenation of surface OH groups: 4(OH),+O2 ---* 4(0),+2H,O. (4) The absence of surface oxygen vacancies in the case of Li,O/MgO treated in H2 at 600°C was proved by re-oxidation experiments with N20 as the oxidant. Under these conditions the amount of N2 is equal to that formed on oxidized Li,O/MgO and corresponds to catalytic decomposition of N20: After additional treatment (at T = 700 "C in He) of the reduced sample the amount of N2 formed in the first pulses was much higher than that for the oxidized or the unheated in He samples of Li2/Mg0. In this case the treatment of the reduced sample causes the dehydroxylation of the sample surface and interaction of N20 with vacancies formed by the treatment takes place The activation energy of the re-oxidation of pretreated in He Li20/Mg0 (100 kJ mol-') and the heat of the interaction of oxygen atoms with the anion vacancies (420 kJ mol-') were determined.Thus, the re-oxidation process going via the formation of vacancies must be more suppressed in the presence of both methane and N 2 0 because of H20 formation which shifts the equilibrium state of reaction (2). This conclusion is in good agreement with the results reported by Prof. Lunsford and co-workers.' They observed a decrease of activity of Li,O/MgO catalyst with respect to CHI, radicals formation in the presence of a CH4-N20 mixture, while the formation of CHI, radicals in the presence of a CH4-02 mixture proceeds with a constant rate. Methane oxidation in the presence of oxygen and nitrogen oxide over a silica gel supported molybdenum catalyst has been rather extensively studied.Formaldehyde, CO, C02, and, in the presence of water, methanol were the basic reaction products. However, the methane conversion and the selectivity of formaldehyde generation were not high. The catalyst structure is known to have a marked effect on catalytic activity in certain oxidation reactions. My colleagues at the Institute of Chemical Physics essentially modified the catalyst structure by mechano-chemical activation.' The systems: SO2-supported Moo3 (I), Moo,-SiO, mixture (11), and a mixture of low-dispersity Mo and Si02 (111), as well as MOO and Si02 oxides were milled in a quartz vibrating mill for times of various duration (table 2).The catalytic activity was studied in a differential reactor in the presence of nitrogen oxide or air at 600 "C and contact time 0.5 s, both before and after activation. The milling of catalysts representing a specific mixture of meta molybendum or of MOO, with silica gel decreases the catalytic activity. For the metal and its oxides mixed with Si02, methane conversion becomes several times lower, but the formaldehyde 2N20 + 2N,+0,. ( 5 ) Li+( - ) + N 2 0 -+ Li'O- + N2. (6)52 General Discussion Table 2. Effect of mechano-chemical activation on the activity of Mo-containing catalysts in methane oxidation (20% CH4, 60% N20, 20% Ar) catalyst ~ selectivity (Yo) activation time CH, conversion (.r)/min (Yo 1 CH2O co co2 2% MoO3/SiOz (supported) 2% MOO,-SiO, (mixture) 10% MoO,-SiO, (mixture) 10% Mo-SiO, (mixture) M003 SiOz 0 180 0 180 0 180 0 180 0 180 0 180 1.3 1 .1 1.2 0.6 2.0 8.1 2.1 1.1 1.2 46 26 28 57 10 33 14 16 70 - - 100 low-active 100 100 - - 9 73 18 - - inactive 26 30 44 26 28 48 " Wt% of Mo in MOO,. generation selectivity attains up to 100°/~. The catalytic activity of silica-gel-supported MOO, does not change upon mechano-chemical activation. As shown by the X-ray method, mechano-chemical activation of samples I1 and I11 results in dispersion of the Mo and MNoO, crystals from 1000 to 300 A, whereas only amorphization of S O 2 occurs for sample I. It can be seen from the TEM patterns that besides being reduced in size, the molybdenum crystals become enveloped by S O , [plates l ( a ) and ( b ) ] .Along with the electron micrograms, the formation of capsules representing Si0,-enveloped Mo particles is confirmed by the results for catalyst 111 interaction with oxygen at 600 "C. With no mechano-chemical activation Mo stoichiometrically converts to MOO,. After 5 h treatment in the quartz mill the uptake of oxygen by the catalyst decreases by a factor of 1.5. As seen from X-ray data, after methane oxidation as well as after interaction with oxygen, the initial catalyst I11 completely oxidizes to MOO, while the mechano-chemically activated samples left in the reaction medium or subjected to oxygen represent different valent molybdenum oxides and metal Mo. It seems that mechano-chemical activation results in specific interaction of silica gel not only with Mo, but also with Moo3 particles and is accompanied by enveloping of these particles with SiOz.As a result of this the active catalyst oxygen contributing to carbon dioxide formation becomes fixed. As found by a specific experiment, CO forms essentially by formaldehyde decomposition. After activation of the catalyst C H 2 0 decomposition markedly diminishes and no CO appears in the reaction products. The absence of such an effect for catalyst I seems to be caused by the fact that, even in the course of the catalyst preparation, there occurs chemical bonding between the two phases responsible for catalytic activity and unaffected by mechano-chemical activation. 1 J. Lunsford et af., J. Am. Chem. SOC., 1985, 58. 2 G. A. Vorob'eva, A. A. Firsova, A. A. Bobyshev and D.P. Shashkin, Mechanical activation of oxide molybdenum-containing catalysts for methane oxidation to formaldehyde. Collected papers of the All-Union Conference on Chemical Syntheses Using Single-carbon Molecules, Nauka, Moscow, p. 78. Prof. J. B. Moffat (Uniuersity of Waterloo, Ontario) said: As all of us know, the earliest work on structure-sensitive reactions was largely concerned with bimetallic surfaces. We recall the impressive work by Sinfelt and coworkers on the Cu-Ni and Cu-Ru surfaces with the hydrogenolysis of ethane and the dehydrogenation of cyclo-Faraday Discuss. Chem. SOC., 1989, Vol. 87 Plate 1. ( a ) Initial mixture of Mo crystals with SiO,; ( b ) after 5 h milling. General Discussion (Facing p. 52)General Discussion 53 Table 3.Catalyst preparation route product precipitation selectivity (YO ) ignition selectivity (% ) ethane 32 9 ethene 31 9 carbon dioxide 23 53 carbon monoxide 14 18 hexane. Although there have been admittedly fewer examples of structure sensitivity in oxidation reactions there is evidence that structure sensitivity may occur in such processes. I recall the work of Tatibouet and Germain on the oxidation of methanol on Moo3 catalysts, in my opinion a convincing demonstration of structure sensitivity. We may also recall the work of one of our colleagues present here today, namely that of Volta and Vedrine, on propene oxidation on MOO,, another unambiguous example of structure sensitivity in oxidation processes. Recent work by Lambert and coworkers on the oxidation of ethene also suggests that structure sensitivity may exist in this process.Furthermore, I believe that Prof. Ponec will show us another example in the selective oxidation of n-butane. It is my understanding, Prof. Lunsford, that you interpret your results as suggesting the absence of structure sensitivity, although you have been careful to note that subtle effects may not have been observed. In view of the aforemen- tioned rather positive examples of structure sensitivity in oxidation processes I am wondering if you would care to speculate for us on the probability that structure sensitivity exists in the oxidative dimerization process. Prof. Lunsford replied: For monophasic oxides structure sensitivity may very well exist in the oxidative dimerization process; however, where the active sites consist of isolated defects or impurity centres it seems that structure sensitivity, in the usual sense, would not be a factor.Dr G. J. Hutchings (Liverpool University) added: In the presented paper it is clear that the addition of Li to the two samples of MgO produces materials that are very similar, and hence it is not possible to make any comparisons with the theoretical predictions of Mehandru et al.' However, in the absence of Li the situation may be less confused since the two MgO samples exhibit very different crystal sizes [MgO(A) 7 nm cubes, MgO (JM) 30 nm cubes]. Table 1 [ref. (l)] gives data for the relative CH; scavenging efficiency for these two MgO samples and it can be concluded that the MgO(A) is superior to the MgO(JM) sample in this respect. Is it possible to comment on the relative reactivity of these two MgO samples for methane coupling since it may then be possible to make some comparisons with the predictions of Mehandru et al.? 1 S.P. Mehandru, A. B. Anderson and J . F. Bradzil, J. Am. Chem. Soc., 1988, 110, 1715. Prof. Lunsford answered: We did not obtain catalytic data on the pure MgO samples, although the comparison you suggest would be interesting. Prof. R. W. Joyner (Liverpool University) made further comments: Prof. Lunsford has presented limited support for structure sensitivity in methane oxidation over mag- nesium oxide catalysts. We have performed experiments on magnesia catalysts prepared in different ways, therefore having quite different morphologies. Catalysts have been prepared by precipitation, which is known to give MgO with a very small crystallite size, (typically <5 nm cubes), and also by ignition of magnesium ribbon in air, followed by calcination to decompose any nitride phase.This latter approach generates much larger, regular cubes, with side 50-100 nm. The selectivity of the two catalysts prepared by these routes is quite different, and is shown in table 3.54 Genera 1 Discussion Electron microscopy of the used catalysts shows that the observed changes in the morphology of the catalysts are only those which are to be expected from the thermal treatment,' and that the reaction gases appear to have little influence on catalyst structure. Magnesium oxide catalysts show high selectivities to molecular hydrogen, (>N% ).Separate studies of the interaction of CO-H20 and C2H4-H20 mixtures with the catalysts have indicated that there are two main sources of hydrogen, the water gas-shift reaction: CO+H20 - CO,+H,, and the steam cracking of ethane to ethene: C2H6 ---* C2H4+H2. Magnesium oxide has no significant activity for the steam reforming of methane or ethane under the conditions of interest, [ T == lo5 K, P(CH,) = 47 kPa, CH4/02 = 5.75, O2 conversion ca. 98%]. These results will be reported in detail elsewhere.2 1 A. F. Moodie and C. E. Warble, J. Crystal Growth, 1971, 10, 26. 2 J. S. J. Hargreaves, G. J. Hutchings and R. W. Joyner, Proc. 2nd Eur. Workshop Methane Activation, to appear in Catal. Today. Prof. M. Ichikawa (Hokaido University, Sapporo, Japan) said: I would like to address your proposed mechanism for the oxidative CH, coupling catalysed on Li+/MgO.In your paper you suggest that the CH; radical, most probably the intermediate for C, hydrocarbons, is directly derived from a homolytic C-H splitting of CH, by 0- sites on the Li+-promoted MgO surface. My questions are as follows: ( 1 ) From a thermodynamic point of view, which is energetically most favourable for the production of CH' radicals: a heterolytic C-H bond activation causing CH, species, which may be converted by electron abstraction (via the redox process), or a direct homolytic H-abstraction from CH, catalysed on an 0- site? Additionally, is there any evidence to find a possible role of CH, species incorporated in the oxidative coupling of CH4? CH, H' CH,+[O-M'] [M ' + -01 ' li-.'I' CH3-CH3 + CH;+[M+-O-] (2) Have you observed any kinetic isotopic effect by using d,-methane or "O-MgO/ Li for the oxidative methane coupling reaction on this particular catalyst or other oxide catalysts such as Y2O3 and Ce2O3? If so, how do you rationalize the data for your proposed mechanism of CH, activation? Prof. Lunsford replied to each of these questions: (1) The heterolytic breaking of the C-H bond by 0- is favoured both by the gas-phase reaction studies of Bohme and Fehsenfeld' and by the theoretical calculations of Mehandru et al. [ref. (16) in the paper]. I have no information on the energetics of the homolytic process which you propose. (2) Although we have not observed a kinetic isotope effect using CH, and CD,, Cant et al.[ref. (14) in the paper] have observed such an effect and the implications are briefly discussed in terms of 0'- or 0, as the actual active site. Another possibility is that a substantial fraction of the CH4 could react via gas-phase chain-branching reactions that are initiated by surface-generated CH; radicals. 1 D. K. Bohme and F. C. Fehsenfeld, Can. J. Chem., 1979, 47, 2717.General Discussion 55 Table 4. yield (YO) CH4 conv (YO) H2C0 CO CH3CI 20% HPMo/Si02 w = 1 g (paper, fig. 5) no TCM 0.35 0.21 0.13 0 0.17 mol% TCM 1.2 0.44 0.54 0.14 no TCM 0.1 0.06 0.032 0 0.17 mol% TCM 0.3 0.03 0.033 0.24 20% HPW/Si02 w = 2 g (fig. 7) Prof. A. K. Datye (University of New Mexico, Albuquerque, U.S.A.) in response to informal remarks made by Prof.P. B. Wells: Prof. Wells suggested that the presence of water may be causing a rounding off of the MgO cubes. However, the phenomenon of thermal roughening alone may account for the rounded corners seen in the used catalysts. In the case of ionic crystals, such as NaCl, Heyraud and Metois' have shown that while the equilibrium shape at low temperatures is a cube, at higher temperatures all the edges and corners become rounded. Comparison of the used catalyst with MgO cubes heated in inert atmospheres may allow a better estimate of the morphological changes caused by the reactive environment. 1 J. C. Heyraud, and J. J. Metois, J. Cryst. Growth, 1987, 84, 503. Dr G. J. Hutchings addressed Prof. Moffat: As noted in your paper the addition of tetrachloromethane during methane activation gives different effects on selectivity for the two structurally related heteropoly oxometallates, which may be indicative of the underlying oxidation reaction mechanism.It is the differences observed for the activity and selectivity under comparable conditions that allows one to rule out the operation of a purely homogeneous gas-phase reaction mechanism on the addition of the halo- compounds. Perhaps the difference in the two systems, i.e. Mo and W, is in the ability to activate the oxygen rather than the methane. Consider the data at comparable conditions, i e . 450 "C, CH4/N20 = 4, F = 60 cm3 min-' (table 4). It is clear that the yield of CH,Cl for the two catalysts is comparable, whereas the overall methane conversions are very different with the Mo catalyst giving much higher conversions.This could suggest that the CH3Cl is predominantly formed via a gas-phase reaction, but that the Mo system activates oxygen more effectively. Do you have any comments on the relative mechanisms of oxygen activation with these two heteropoly oxometallates ? Prof. Moffat replied: Work in our laboratory' with heteropoly oxometallates pre- treated with tetrachloromethane (TCM) prior to conversion of methane in the absence of TCM has shown that, at least initially, the observations are similar to those found where a small quantity of TCM is added continuously to the feed stream. This suggests that the catalyst is participating in the process involving chlorine-containing species and that chlorine in as yet an unknown form is incorporated on and/or in the solid.Earlier EXH calculations from this laboratory on heteropoly oxometallates of Keggin structureL have predicted tht the terminal oxygen peripheral metal bond of the heteropoly anion is weaker in the molybdenum-containing anions than in those containing tungsten. These results are consistent with those from earlier temperature-programmed desorption, exchange and reduction experiments.3 The evidence for the importance of the proton as shown in the present work has been interpreted as related to the formation of oxygen vacancies resulting from the extraction of oxygen atoms from the anion by the protons.56 General Discussion In view of the aforementioned, the formation of vacancies may be predicted to be more facile with molybdenum-containing than with tungsten-containing anions.Furthermore, it is suggested that the presence of vacancies, at least initially, is necessary for the oxidation process. Thus it would appear that the molybdenum heteropoly oxometallate should be more active in the processes involving the transfer of oxygen. 1 S. Ahmed and J . B. Moffat, J. Catal., in press. 2 J. B. Moffat, J. Mol. Catal., 1984, 26, 385. 3 B. K. Hodnett and J . B. Moffat, J. Catal., 1985, 91, 93. Prof. J. Haber (Polish Academy of Sciences, Krukow, Poland) then said: In your model of the interaction of methanol with the Keggin unit you are assuming that the methoxy species are formed with the participation of the terminal an-bonded oxygen atoms. Intuitively such an assumption may seem plausible; however, more detailed discussion leads to the conclusion that it is rather the bridging oxygen atom which is the most probable site of the reaction.The following arguments may be raised in favour of such an alternative: (i) the HOMO of the Keggin unit is mainly composed of lone-pair orbitals of the bridging oxygen atoms.' (ii) due to the distortion of the octahedron resulting from the shift of the metal centre towards the terminal oxygen atom the latter becomes more acidic, whereas the bridging oxygen atom becomes more basic;' (iii) 13C n.m.r. spectra recorded after interaction of methanol with the Keggin unit point directly to the bridging oxygen atom as the site where CH3 group is linked;' (iv) "0 n.m.r. spectra of 12-molybdophosphate solutions indicate that electron- density charge on bridging oxygens is higher than on terminal ones.4 1 Taketa, S. Katsuki, K.Eguchi, T. Seiyama and N. Yamazoe, J. Phjas. Chem. 1986, 90, 2959. 2 J. B. Goodenough, Proc. 4th Int. Con$, Chemistry and Uses of Molybdenum, Golden Colorado 1982, ed. 3 W. E. Farneth, R. H. Staley, P. J . Domaille and R. D. Farlee, J. Am. Chem. SOC., 1987, 109, 4018. 4 R. I. Maksimovskaya, M. A. Fedotov, V. A. Mastkhin, L. I . Kuznetsova and K. 1. Matveev, Dokl. Akad. H . F. Barry and P. G. H. Mitchell (Climax Molybdenum Co., Ann Arbor 1982), p. 1. Nauk SSSR, 1978, 240, 117. Prof. Moffat, in reply to this, then said: Photoacoustic FTIR spectra obtained in this laboratory have shown that methanol is protonated at room temperature by such heteropoly oxometallates as 12-tungstophosphoric acid.On stepwise heating to 150 "C the bands associated with protonated methanol diminish in intensity and a sharp band develops at 1453 cm-' and progressively increases in intensity. This band is attributed to the CH3 symmetric deformation in the CH30 group. Subtraction of the room- temperature spectrum from that of the heat-treated sample (fig. 3) revealed the progress- ive development of a band at 1022cm-] (inset, fig. 3 ) which appears to relate to the progressive formation of a metal alkoxide-type structure. For example, the C -0 stretch in W(OCH3)6 appears at 1070cm-'.' These observations suggest that the methyl group [CH:] is attached to a terminal oxygen atom of the heteropoly anion. Earlier EXH calculations from this laboratory predict that the magnitude of the charge on the terminal oxygen atom should be higher than that on the bridging oxygen atom of the anion.? It should be noted, however, that attack of a methyl group on a bridging oxygen atom of the anion may lead to a scission of one of the W-0 bonds of the bridge, leaving the methyl group apparently bound to a terminal oxygen atom.However, it seems more reasonable to assume that the methyl groups may be bound to both terminal and bridging oxygen atoms of the anion and a steady state or equilibrium may develop between the methyl groups associated with these two centres.General Discussion 57 (el (dl + CH30H at 25 "C 0 I 1 1 I I I 2000 1600 1200 800 wavenumber/cm-' Fig. 3. Effect of stepwise heating in U ~ C U O on spectrum of 'irreversibly sorbed' CH,OH on 12-tungstophosphoric acid.( h ) 50, ( b ) 70, ( c ) 110, ( d ) 150 "C (inset peak obtained by subtraction of spectrum of pre-evacuated acid, normalized at 1080 cm-'; ( e ) effect of dosing ( d ) with excess CH,OH at 25 "C and evacuation at 25 "C. 1 D. C. Bradley, M. H. Chisholm, M. W. Extine and M. E. Stager, Inorg. Chem., 1977, 16, 1794. 2 See, for example, J. B. Moffat, in Prepararion of'Caralysts IV. Srud. S u r - Sci. Catal., ed. B. Delmont, P. Grange, P. A. Jacobs and G. Poncelet (Elsevier, Amsterdam, 19871, vol. 31. Dr E . M. Senvicka (Polish Academy of Sciences, Krakow, Poland) said: Regarding Prof. Moffat's paper, I would like to comment on the nature of surface species responsible for catalytic activity in methane oxidation.Optimum temperature reported here for this reaction seems to exceed considerably the stability range known for heteropoly acids. It is therefore conceivable that Keggin anions act as a precursor of an active phase rather than survive in situ under reaction conditions. Results obtained in our laboratory seem to confirm such a hypothesis. Fig. 4 shows the i.r. spectra recorded for 0.1 monolayer H,PV2Mo,,,O,,,/SiO2(400 m' g-I) catalyst subjected to various treatments, after subtrac- tion of the silica background. The fresh sample (solid line) displays all the bands expected for the Keggin anion uiz. 785,865,960 and 1065 cm-'. The spectrum recorded immediately after 4 h calcination in air at 673 K (dashed line) shows that the Keggin structure has collapsed, to be replaced by decomposition products, possibly a mixed molybdenum-vanadium oxide system, with characteristic bands at 830 and 1015 cm- I .Simultaneously, the originally yellow sample turns white. However, on exposure to air (ca. 70% natural humidity), at room temperature, the sample recovers most of its original colour within half an hour, and corresponding changes in the i.r. spectra follow (dotted58 * . J I I I General Discussion 700 800 900 1000 1joo wavenumber/ cm-’ Fig. 4. 1.r. spectra of 0.1 monolayer H5PVzMo,0040/Si02 (400 mz g-’). (-) As prepared, yellow; (- - -) immediately after calcination in air at 673 K for 4 h, white; (. * - - .) after calcination in air at 673 K for 4 h stored for 0.5 h open to air at room temperature, yellow.line). It is obvious that a swift reconstruction of Keggin units takes place at room temperature under ‘moist’ conditions. This result indicates that one has to be very cautious about concluding on the surface composition of a working catalyst on the basis of investigation of a spent catalyst only. Examination in situ seems necessary for an unequivocal identification of the catalytically active phase. Prof. Moffat responded: Results from our laboratory have provided information on the nature and properties of catalysts prepared by supporting heteropoly oxometallates on a support such as silica.’.’ The observation that the rates of formation of products in the conversion of methane at 843 K increase linearly with loading of 12-molybdophos- phoric acid [H3PMo,2040, abbreviated to HPMo) on Si02 and at low values of loading extrapolate to those results found for the support itself clearly indicates that the active species are associated directly with the supported materials (fig.5). The effect of the temperature and duration of pretreatment on the activity, selectivity and remaining molybdenum loading of a 23 wt% HPMo/SiO, catalyst provides evidence for the effect of the support in the enhancement of the thermal stability of the heteropoly oxometallate (fig. 6). The conversion and selectivities in the oxidation of methane are approximately constant for pretreatment temperatures up to 773 K and for higherGenera 1 Discussion 59 H3PMo12040 loading (wto/o 1 501 I 0 0.9 - I v) -w 0.7 E - l 0, ;2- 0.5 2 c) c 0 .- Y < 0.3 E 0 .I 0 0 100 20 0 300 H3PMo,,0,0 loading/ mol KU g-' Fig.5. Effect of the HPMo loading of the support on the production rate of the different products of the CH4-N20 reaction at 843 K. Reaction conditions: CH4 (67%), N 2 0 (33'/0), W=OS g, F = 30 cm3 min-'. ( x ) N2, (+) total carbon detected, (a) HzO, (V) CH30H, (A) CO, (0) CO:, (0) CH20.60 General Discussion I I I * 1 I I I 1 J O $ , 60 0 80 0 1000 0 20 LO 64 80 100 - 20 - - 10 - calcination temperature/ K time of calcination/h Fig. 6. Effect of the temperatures of calcination over 16 h (left) and of the time of calcination at 823 K under air (right) on the CH4 conversion, selectivity, and Mo loading of the 23-HPMo catalyst. Reaction conditions: CH, (67%), N 2 0 (33%), TR = 843 K, W = 0.5 g, F = 30 cm3 min-'.( A ) CO, (0) COz, (0) CH20, ( x ) CH, conversion, (0) Mo loading. temperature the conversion decreases sharply, while the production of CO and CO, remains constant up to 900 K. Above 900 K the production of H2C0 and CO decreases, while that of C02 increases, all three apparently approaching the values expected for the silica support alone. It is evident that the activity of the HPMo catalysts can be related to the presence of a thermally sensitive species whose degradation products have a substantially reduced activity in the oxidation of methane and the thermal stability of the HPMo is enhanced by the presence of the support. The existence of PMo, ,O& anions with Keggin structure on various silica-supported samples was confirmed from the infrared spectra of solutions resulting from the washing of the samples with acetonitrile (fig.7). The characteristic bands of the Keggin structure at 1080 and 969-960 cm-' are still present and intense even after heating at temperatures up to 923 K for 16 h in air, although some diminution of the intensity of the bands can be observed. These observations clearly demonstrate that the Keggin structure still exists even after such vigorous pretreatment as heating in air at 913 K for 16 h. Raman spectra of silica-supported 12-molybdosilic acid ( H,SIMoI2O,,,) show the presence of the Keggin structure even after heating to 773 K for 2 h in air (not shown, see fig. 2 in comment on Bond paper p. 102). 1 J. B. Moffat and S. Kasztelan, J. Curd., 1988, 109, 206. 2 S. Kaszteian, E. Payen and J .B. Moffat, J. Card., 1988, 112, 320. Dr R. Burch (University of Reading) addressed Prof. J . M. Thomas: In your paper on layered oxychlorides for methane activation you show that for certain catalysts there is a very high ethene/ethane ratio. It is now fairly well known that even a smallGeneral Discussion 61 1063 969 883 1200 1000 800 wavenumber/ cm-' Fig. 7. Infrared spectra of acetonitrile solution after washing of the following supported HPMo samples calcined under different conditions. ( a ) Bulk H3PMo,204; ( b ) 1.16 HPMo, 350 "C, 2 h; (c) 11.1 HPMo, 350 "C, 2 h, then 20.1 HPMo sample; ( d ) 350 "C, 16 h; (e) 450 "C, 16 h; (f) 550 "C, 16 h; (g) 640 "C, 16 h; ( h ) 640 "C, 16 h, followed by a rest at 570 "C, 10 h; (i) 730 "C, 16 h. concentration of chlorine radicals in the gas phase can catalyse the rapid dehydrogenation of ethane to ethene, particularly in the presence of residual oxygen.Since in your experiments you are using quite large reactors and quite slow volumetric flow rates the residence time of the primary product (ethane) will be long. It seems quite possible, therefore, that much of the ethene observed could be formed by a purely gas-phase radical-catalysed dehydrogenation of ethane. Do you have any evidence that ethene is produced on the surface of your catalysts? Furthermore, do you have any information on the amount of chlorine being released from the various catalysts used in your work and is there any correlation between the rate of loss of chlorine and the ethene/ethane ratio?62 35 30 25 5 20- ..$ 15- 10 Genera 1 Discussion - - - - 5 - 0 080 I L 0 0 Fig. 8. Correlation between C2H4/C2H, ratio and CH4 conversion, data taken from table 1 . Dr G. J. Hutchings made the next comment regarding Prof. Thomas' paper: The catalytic performance of LiCa2Bi3O4CI6 for the methane-coupling reaction is indeed most promising. This compound contains the X3 structural unit and possesses the highest activity for the production of C2 hydrocarbons of all the oxyhalide structures investigated. It is interesting to compare the activity of this catalyst with that of catalysts previously researched. Based on a surface area of 1 m' g-' and using the data in table 1 of the paper it is possible to calculate a specific activity for LiCa2Bi304C16 of 7.8 x mol C2 m-2 h-' at 700 "C and 26 x mol C2 m-2 h-' at 720 "C, for a CH4 feed-rate of 132 x mol m-2 h-'.This is comparable to the specific activity of 15% Na/CaO' which is 10.4 x mol m' h-'. On this basis it is possible to place LiCa2Bi304C16 into the activity series that has been calculated for methane-coupling catalysts' Li/Sm203 > LiCa2Bi304C16 = Na/CaO > K/CaO > Sm2O3 = Li/CaO > Li/MgO > MgO. It is therefore apparent that this compound is a particularly active catalyst, and further modifications could improve its specific activity. The extremely high ethene/ethane ratio observed with the oxyhalide catalysts is also most interesting. A number of reactions for ethene formation from ethane are possible: (i) non oxidative dehydrogenation, (ii) oxidative dehydrogenation, and (iii) steam cracking.On inspection of the data in table 1 of the paper it would appear that there exists a correlation between the C2H4/C2H6 ratio, observed for the oxyhalide catalysts, and either the methane or oxygen conversion (fig. 8). Since the concentration of water increases with increasing conversion, then it is possible that the high ethene/ethane ratios could be due to homogeneous gas-phase ethane steam cracking to ethene. Van Kasteren et al.3 have commented that the post-reactor heated volume is a critical parameter controlling the ethene/ethane ratio. It is possible to comment on the possibil- ity of such secondary reactions occurring. 1 C. H. Lin, T. Ho, J-X. Wang and J . H. Lunsford, J. Am. Cbem. Soc., 1987, 109, 4808. 2 G. J.Hutchings, M. S. Scurrell and J. R. Woodhouse, Chern. Soc. Rev., 1989, 18, 251. 3 H. M. N. van Kasteren, J. W. M . Geerts and K. van der Wiele, Proc. 9th Int. Congr. Catal., ed. M. J . Phillips and M . Ternan (Chem. Inst. Canada, 1988), 2, 930.General Discussion 63 Prof. J. B. Moffat had three questions for Prof. Thomas: Prof. Benson has shown that the process in which a chlorine atom extracts a hydrogen atom from methane Cl+CH4 + HCl+CH, is a fast reaction and produces the expected HCl. You have carefully noted that it is not clear whether or not chlorine is removed from the catalyst during the process. Do you have any additional information in this regard concerning the deactivation of the catalyst? You have also commented on the possibility that less labile chlorine atoms are preferred.Do you have any further information on this topic? Have you tested for chlorine and/ or chlorine-containing species in the product? Prof. J. H. Lunsford had similar questions: From a technological standpoint the production of ethene, rather than ethane, is an important consideration in the oxidative dimerization of methane. The two catalysts, NaCa2Bi3O4Cl6 and LiCa2Bi,0,C1, are therefore most interesting because of the very large C,H,/C,H, ratios (table 1 in paper) that were obtained. Otsuka et al.' have previously observed large C2H4/C2H6 ratios over LiCl/ Ni-oxide and LiCl/ Mn-oxide catalysts; however, the C2H4 selectivity decreased sharply after 2 h on stream. These authors suggested that chlorine atoms in the catalysts diminished with time by evaporation, decomposition and reaction with the reactants. Do you have any evidence that chlorine atoms play a role in your catalytic system to give the unusually large C2H4/C2H6 ratios? For example, to what extent is chlorine lost from these catalysts and is there a concomitant formation of chlorinated hydrocarbons? As the activity of catalyst 2, table 1, decreased and the selectivity increased, did the C2H4/C2H6 ratio change significantly? 1 K.Otsuka, Q. Liu, M. Hatano and A. Morikawa, Chem. Lert. 1986, 903. Prof. Thomas replied to all these questions: Drs Burch, Hutchings, Moffat and Lunsford all raise important issues concerning our bismuth oxyhalide catalysts. The key points are worth repeating, namely that the activity of these monophasic solids is about as high as that of the best catalyst so far reported for the oxidative coupling of methane. And the ethene to ethane ratio is very high. We have recognized from the outset' that the role of chlorine atoms could be critical in the good performance of our catalysts. Indeed it was a deliberate act on our part to insert both C1 and Bi (each being well known hydrogen extractors) into the designed solid catalyst with the explicit intention of engendering conditions conducive to the production of CH, and other radicals, which would then produce desirable products either homogeneously in the gas phase or heterogenously through the agency of the surface of the catalyst. The possibility that homogeneous, gas-phase steam cracking of ethane to ethene also occurs must certainly not be discounted. Dr Hutchings is right to draw attention to the critical role of the post-reactor heated volume, the point first highlighted by van Kasteren et al. We do know, and this has been emphasized already,"2 that halogen tends to be lost from catalysts such as LiCa,Bi304C1, and BiOCl under the operating conditions described in our paper. The degree of loss varies considerably depending upon the structure of the catalyst. Whilst we have not made a systematic study of the relationship between chlorine loss and catalyst performance, we do know that some oxhyalides lose their halogen much less readily than others during use. In work carried out since our paper was written, we have prepared, and studied the catalytic performance, of monophasic solid solutions of BiOCl and LaOCl as well as BiOCl and SmOCl. The results, which will be described el~ewhere,~ are encouraging so far as the stability of these solid solution64 General Discussion catalysts is concerned. But the ethlene/ethane ratios fall as a result of replacement of Bi by either La or Sm. More work using other techniques is clearly called for. 1 W. Ueda and J. M. Thomas, Proc. 9th Congr. Card., Calgary, 1988, 2, 960. 2 W. Ueda and J. M. Thomas, J. Chem. SOC., Chem. Commun., 1987, 19. 3 J. Williams, J. M. Thomas, J. Kent and R. H. Jones, in preparation.