H.E. Curry-Hyde and R.F. Howe (Editors), Natural Gas Conversion 11 0 1994 Elsevier Science B.V. All rights reserved.
Isotopic exchange and volumetric studies on methane activation over rareearth oxides. S. Lacombe, A. Holmena, E.E. Wolfb, V. Ducarme, P. Moral and C. Mirodatos. Institut de Recherches sur la Catalyse, 2 Avenue Albert Einstein, F-69626 Villeurbanne Cedex, France. aDepartment of Industrial Chemistry, University of Trondheim, N-7034 Trondheim, f;lonvay. University of Notre-Dame, Notre-Dame In., USA.
A series of rare-earth oxides (REO) has been tested towards the CH4/CD isotopic equilibration and the oxidative coupling of methane (OCM): a straightforwart relationship was found between the ionic radius of the catalysts and the reaction of equilibration but not with the OCM reaction. No significant methane adsorption was detected by volumetry, but methane reforming into syngas by reaction with surface carbonates was observed at hi h tem erature in the absence of oxygen. These results are discussed within the scope oft e OC mechanism in relation with catalyst basicity.
1.INTRODUC"ION In the frame of the oxidative coupling of methane, it has been roposed as a general rule that increasing the basicity of active oxides im roves the 2 + selectivity and yield at the expense of the undesired COXproducts (1). l!owever, if basicity may be considered as an important feature for successful catalysts, still highly basic materials may be poor OCM catalysts. Thus, within the sequence of lanthanide oxides for which basicity can be related to the ionic radius, Otsuka et al. (2) have observed that neither the methane conversion nor the C + selectivity followed the ionic radius/basicity sequence. Similarly, Tong et al. (3) zave shown that the rare-earth oxides were good methyl radical generators except for the three elements Ce, Pr and Tb, the reverse being observed for the combustion of the radicals by these oxides. This effect was assigned to the extensive ability of the reducible oxides to react with CH3 radicals via the reductive addition rea tionCH3 + L8 + 0 2 - --- > L3 + (0-CH3)- --- > --- > COX 111
The direct involvement of basicity in the OCM reaction has been formalized through the mechanism based on original ideas of Stone et al. (4) and cently proposed by Ito et al. (9,assuming that methane is initially activated on basic 0 atoms having a low coordin ion number: ---> CH7- + OH121 CHA+ followzd by an electronJabstractionby gaseous oxygen, leading to methyl radicals: CH3- + 02---> CH3' + 02[31
In order to precise this have investigated the reaction oxides, in the absence and in been proposed to proceed reflecting the surface basicity (6).
basicity and methane activation, we of lanthanide ration reac 'on has on Mn+-$- sites,
Volumetric measurements have also been carried out under the conditions of the isotopic equilibration in order to check if a significant methane uptake occurs on the oxide surface, which remains a subject of much discussion (6-10). 2. EXPERIMENTAL
A series of rare-earth oxides was provided b Rhbne-Poulenc (purity 99%). Their surface area was determined in a special BET cel allowing in sQu pretreatment under reaction conditions. The CHqlCDq equilibration has been carried out in a dixed bed quartz microreactor at at 0s heric pressure in the temperature ran e 600-950 C, with a total flow-rate of 1.8 1 h-T i f t h e absence of oxy en (CHq/CDq/t8e = 4/4/92) and in the presence of oxygen (CH4/CD4/02/He = 4 4/4/88). Gas analysis was performed by on-line mass spectrometry or after GC se aration (in order to suppress any contribution of water, C2 and CO fragments in t e amu range 12 to 20, corresponding to the isotopic distribution of metianes). The degree of exchange ( 0 % < a <100%) was calculated from the normalized difference between the actual and the statistical isotopic distribution. Volumetric measurements were carried out by introducing a calibrated amount of pure methane (ground lml/NTP) in a static vessel containing a sample previously outgassed at 750 C (around lg). In order to detect eventual changes in gas composition after methane admission, the same experiments were carried out allowing a continuous leak into a mass spectrometer.
3. RESULTS 3.1. CH4ICD equilibration. In the d s e m e of oxygen, the extent of CHq/CDq equilibration was determined as a function of temperature (Fig. 1); as a common feature, the de ree of exchange increases with temperature, reaches a maximum which may be lower t an the statistical equilibrium ( a =100%) and decreases for hi her temperature. In the first part of the curves, the (Begree of exchange remains low.and mostly stepwise (essentially CH3D and CD3H are appearing). The a arent activation energies corresponding to this low conversion range are reported in Ta e 1.
Table 1 Apparent activation energy of the equilibration reaction carried out in the absence of oxygen, and BET surface area measured after equilibration reaction, carried out at the temperature (re orted ipto Jmckets) corresponding to the same intrinsic rate of exchange (8.4 10' mol h- m- ).
E (kcal/mog area (m /g) (T/ C)
38 4.0 (700)
44 3.1 (712)
42 2.3 (750)
37 5.2 (720)
38 4.7 (675)
38 5.8 (780)
33 2.0 (800)
The rather close values of E obtained for all the catalysts in the absence of oxygen tend to indicate that the e ufibration proceeds via the same mechanism on the whole series of rare-earth oxides. iowever, it appears clearly in Fip. 1 that the exchange curves are markedly shifted as a function of tem erature, dependin on the considered ! I es towards CH4 CD4 equilibration oxide. In order to rank precisely these oxi (accounting for the actual surface area under reaction conditions), t e temp rat e at which each catalyst presented a given intrinsic activity, fixed at 8.4 1Os pol h-? m y has
been determined. This reference value of the intrinsic activity has been chosen in order to correspond to degrees of stepwise exchange smaller than 30%. These temperatures of isoactivity are reported in Fig. 2 as a function of the ionic radius for the tested oxides (for the reducible elements, the different possible radii are reported). 100 I
Figure 1. Extent of CH4/CD equilibration (a) in the absence o? oxygen versus temperature.
Figure 2. Temperature of isoconv sion in CH /CD4 equilibration (8.4 10- mol h-* m-$ vs ionic radius.
If one considers he rare-earth oxides in their highest possible oxidation state (e.g. Ce4+ for ceria and Smf for sarnaria), which corresponds to their smallest ionic radius, a clear correlation is observed between the intrinsic reactivity for CH4/CD equilibration and the R E 0 ionic radius. In contrast, no correlation can be founi considering the reduced form of the oxides. As mentioned above, it has also been observed that for most of the studied oxides there was a temperature above which the equilibration rate started to decrease (Fig. 1). Two types of deactivation were observed: i) slow deactivation for the non reducible oxides such as lanthana and for the ones with 3 + / 2 + valencies such as samaria, still keeping a si nificant exchange ca acity at high temperature, ii) fast and deep deactivation or the reducible oxi es with 4 + / 3 + valencies, the highest rate of deactivation being observed tor the ceria catalyst which deactivated from a =95% to a=7% in few minutes at 840 C. For boJh cases, the catal sts were totally regenerated after a treatment under oxygen at 500 C; release of C 2 was observed during this regeneration, es ecially for the case of ceria. From these experiments, it can be assumed that the reversi le deactivationjs mainly due to poisoning of the exchange sites by carbon deposits, oxidable at 500 C. This carbon deposition would arise from methane decomposition (thermal cracking), favoured for the case of ceria where a bulk reduction 4 Ce02 + CH4 ---> 2 Ce2O3 + C + 2 H 2 0 [41 could account for the large amount of deposited carbon. These results strongly sug est that the R E 0 resenting variable valency are active for the isotopic exchange when t ey are oxidized; t is feature agrees with the fact that the correlation in Fig. 2 considers only the ionic radius of the oxidized catalysts. +
In the presence of oxygen, i.e. under OCM conditions, the CH,/CD4 equilibration rgte was significantly decreased, as reported in Table 2 for the lanthana catalyst at 750 C (a varies from 10.9 to 6.0 % after oxygen addition). Note that under these conditions, a large part of the methanes was converted into OCM products !xCH4 + Cp4 35 %), which resulted both in a lower partial ressure of methane and in a partia pressure of C 0 2 of around 2.1 kPa in the cataytic bed. It can also be =&
mentioned that the light methane was around 1.7 time more converted into OCM products than the deuterated one, attesting a marked kinetic isotopic effect, as generally observed for this reaction (8,ll). Table 2 Effect of oxygen and carbon dioxide on the CHq/CDq equilibration at 750'C (m= 19.6 mg, flow rate= 1.8 Vh). ~~
CH4/CD4/He 10.9 CHq/CD4/0 /He 6.0 CHd/CDd/C - 2/He 0.8
a = degree of isotopic exchange, XCH4+CD4 = conversion of methanes into C2 and COX,Yc2 = C2 yield.
In the presence of C02 (20 kPa added to the CH /CD4/He mixture), the extent of equilibration is divided by around ten, which clear1 demonstrates that CO acts as a poison towards methane equilibration. This confirms t at this reaction procee s on basic sites, which can be neutralized by the acidic C 0 2 for forming surface carbonates (78).
Performances obtained under OCM conditions for the studied series of lanthanides (conversion of methane and C2 yield) are reported in Fig. 3 as a function of the ionic radius corresponding to the hi hest possible oxidized state, in order to com are straightforward correlation is observe$ as with the CHq/CDq equilibration. originally pointed out by Otsuka et al. (2). It is also well confirmed that the reducible elements (able to change from 4+ to 3+ valencv like Ce and Pr) are active for methane
, Fi ure 3. OCM performances at 750'C, a: 5 ~ ~b: Sc2. 4 , c: Yc2, vs ionic radius.
Figure 4. Changes in the amount of gas (mllg at) in the volumetry cell vs time at 750 ?with La203 (a) and Sm2O3 (b).
3.2.Volumetric measurements. Volumetric measurements 'were carried out on La203 and Sm2O3 at temperatures between 600 and 750 C. In all cases, the pressure was found to increase slowly for several hours after methane admission, as depicted in Fig. 4. Note, however, that a slight transient decrease of pressure was observed for samaria, which could suggest some very weak initial methane uptake for this case. The total amount of the
evolved gases responsible for the ressure increase was calculated by extra olating at infinite time via the transform Ln( ) versus l/t. The corresponding values ax - are reported in Table 3. These evolved gases were found to be mainly CO and H a y mass spectrometry, with a marked trend to get more H2 than CO.
Table 3 Amount of gas produced after CH4 admission on outgassed samples in volumetric measurements.
La23 Sm2 3
slightly on samaria. large amounts under carbonatation before methane conditions the surface carbonate of several ones such as :
amount of reacted carbonate is equal to a third of the pressure increase. For lanthana, this gives around 6 pmol/m 2, a figure which compares quit well with the estimated surface concentration of carbonate species (around 4 pmol/m ). Other reactions such as methane thermal cracking into carbon deposits and hydrogen (which would account for the observed surstoichiomet of hydrogen) : n CH ---> C,H + (4n-x)’h H [91 could &viously ako be involvecfin this process, as it has been shown previously that deactivation via carbon deposition started to develop under similar conditions.
4. DISCUSSION and CONCLUSIONS
Due to the fact that the ionic radius of the studied R E 0 reflects the basic strength of their surface, the present work has first underlined the absence of a direct correlation between OCM performances and surface basicit (Fig. 3). It has also been shown that no significant accumulation of reversibly adsorbelmethane occurred on R E 0 (lanthana and samaria) at high temperature, in accordance with kinetic results derived from isotopic transients studies (10-12). These features tend to be contradictory to i) the OCM mechanism which assumes a first step of methane dissociation on basic sites, followed by a slow step of electron abstraction to form methyl radicals, as proposed by Ito et al. in (3,ii) any OCM
mechanism of Langmuir-Hinshelwood type assuming a first step of methane and oxygen adsorption, followed by a slow surface reaction between adspecies. The Eley-Rideal type mechanism which ostulates the reaction of gaseous methane with adsorbed oxygen, originally propose by Lunsford ( 14), remains therefore the most likely route for methyl radical generation. The second important feature revealed by this work concerns the reversible activation of methane allowing the isotopic exchan e. The single-step character of this exchange and the straightforward correlation with t e RE0 ionic radius, therefore with surface basicity, reinforces the idea of an heterolytic and reversible splitting of CH4 on basic sites accord' to: CH + Lnn+Ofg< = = = > CH -Ln(n-l)+ + OH[lo1 The inhibiting effect of C& (either introduced in the reacting mixture or coming from the total oxidation of metchane in the presence of oxygen) strengthens this correlation between heterogeneous splitting and surface basicity. It is therefore su 4ested that under OCM conditions, beside the OCM route described above (Eley- ideal mechanism), a parallel route allowing a reversible methane activation (fast equilibrium which does not lead to methane accumulation on the surface) also develops on the basic surface sites in competition with C02 (re)adsorption. Note also that in the absence of oxygen (which may be the case under OCM conditions when the 0 2 conversion is completed), toxic carbon species can be deposited, able to poison this reversible activation of methane. Finally, volumetry experiments have revealed that some reforming of methgne into syngas by reaction with surface carbonates could occur at high temperature (750 C) in the absence of oxygen. This unexpected route (observed under static conditions) should be considered In the open field of research on methane reforming.
ACKNOWLEDGEMENTS. Part of this work has been supported b the European Community (Joule I pro ramme). Thanks are due to G.A. Martin and .M. Swaan for helpful discussions an to V.C.H. Kroll for valuable computing and technical contribution.
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