A. Holmen et al. (Editors), Natural Gas Conversion 0 1991 Elsevier Science Publishers B.V., Amsterdam
COMPARISON OF THE PROMOTED ALKALINE EARTH OXIDES CATALYSIS FOR
THE OXIDATIVE COUPLING OF METHMEL* Mgo, cao, srco3, AND Baas SYSIEM Ken-ichi AMA and Takahlto NISHWAMA Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 227 JAPAN SUMMARY Catalytic performances of two gram of MgO, CaO, SrC03 and BaC03 catalysts each of which was promoted with various oxides were compared under the same reaction condition. MgO and CaO systems were much more active than SrC03 and BaCO3 systems. Synergetic effects were observed for MgO and CaO systems when alkali metals were added, while no such effects were observed on SrC03 and BaCO systems. The activity reached a maximum for the samples with small specidc surface area of the MgO and CaO systems, while it was so on the sample with high specific surface area for SrC03 and BaC03 systems. Factors which control the catalyst performance are discussed in two points; chemical factor and physicel factor. CaO doped with alkali oxides gave the highest activity and C2 selectivity among the alkali doped alkaline earth oxides. INTRODUCTION Oxidative coupling of methane has been studied for many kinds of oxides doped with various elements. Especially, alkaline earth oxides, lanthanides, PbO, MnOp systems have been studied in detail 11-17]. Recently, alkaline earth oxide systems have been studied in order to clarify the nature of active sites and the reaction mechanism [1,5-171. These data have been used for the economic evaluation of this reaction process. However, no work to compare the four alkaline earth oxides systems under the same reaction condition has been reported. O u r laboratory has published work done on MgO systems [5,9,12,14] and SrCO3 systems 16,141. Limited work on CaO 1151 and BaC03 systems (61 have been published by u s Here, we obtained some new results about CaO and BaCOp The purpose of this paper is to compare the main results of the four systems and to discuss the catalytic performances with relation to the chemical nature of alkaline earth oxides, and finally to find the factors controlling the activity and the selectivity. ExpERIMprrAL
The important point of this work is to get the data using catalysts with the same preparation method, using the same reactor under the same reaction condition. The catalysts were prepared by the impregnation of MgO, CaO, SrC03
and BaC03 with various metal nitrates in water. Doped amount was usually 5 or 10 mol%. SrC03 and BaC03 were used instead of SrO and BaO, because these oxides might react with the qualtt glass and because oxides and carbonates w o d d give the same surface (mixture of oxides and carbonates) during the reaction (131. Each sample of 2g was evacuated at 773 K for 1 h and subsequently a t 1073 K for 2 h and then used for the reaction. The reaction was performed in a conventional flow system under the same condition; reaction temperatures between 623 to 1073 K, and flow rate of 1.5/3.75/50 ml/min with respect to CHq/air/He gases (CH4/O2 ratio is 2). BET measurements were done after pretreatments using Np XRD measuments of the sample after the pretreatment were conducted by Rigaku Geigeflex RAD-B system.
RESULTS Comrersion vh specific rurface area
Doping of alkaline earth oxides with other oxides generally reduces the specific surface area after calcination (121. For MgO and CaO, the lattice defects formed by the doping Is considered t o be the cause of the sintering. For SrC03 and BaC03, the foreign oxide is considered to be separated out of the host Sr or Ba carbonate because of the large difference in the cation radii. The separated dopant is observed by SEM to cover the host SrC03 and t o stick to each other, which causes surface area decrease (161. Methane conversion a t 1023 K over doped catalysts are shown in Fig. 1 as a function of specific surface area. All the doped MgO and CaO catalysts gave high methane conversion (30 to 40%) and 0 2 conversion (around 95%) indicating the catalysts are so active that only a part of the catalyst is used for the reaction a t 1023 K. On the other hand, methane and O2 conversions increase with an increase of specific surface area for the case of doped SrC03 and BaC03 catalysts. This indicates that these samples are quite inactive and that the dopant and SrC03 give no synergetic effect 1161. Roughly speaking, the activity is linearly related with the surface area, but is not related with the species of dopant.
Fig.2 shows the Cz yield as a function of the specific surface area over doped MgO, CaO, SrC03, and BaC03, respectively. On MgO and CaO systems, the highest C2 yields were obtained a t the smallest surface area Since CH4 conversion is almost the same by changing the specific surface areas, the C2 selectivity is high over MgO and CaO systems with small specific surface area. This phenomenon has been also confirmed for an alkali doped MgO by changing the specific surface area through the calcination at various temperatures . We got higher C2 yield over the catalyst with a smaller surface area On the
t 58a-lOLi 10Ba-1OLi
area I m2g-1
4 6 area I m2g-'
10 surface area
20 I m2g-1
Fig. 1 Methane conversion at 1023 K over various alkaline earth oxides catalysts as a function of the specific surface area. White circle indicates sample without dopants. Numbers indicate mol% of the dopant element. Elements without numbers indicate 0.2 mol% for MgO system and 10 mol% for CaO, SrC03, and BaCOg,respectively.
other hand, on SrC03 and BaC03 systems, C2 yield increases as the specific surface area increases. This is because CH4 conversion increases with the increase of the specific surface area without changing the selectivity much. Strictly speaking, the C2 selectivity was also found to be high on the sample with a small specific surface area in these case, too.
XRD studies disclosed that the alkali doping to MgO and CaO caused
structural changes (the lattice distortion) of the host oxides. These changes are considered to be related t o the activity of C2 hydrocarbons formation. The activation of C-H cleavage may be increased over the surface with lattice distortion which produces an active oxygen. This factor (chemical factor) will be discussed together with another factor, surface area reduction (physical factor), in t h e discussion. On the other hand, XRD study on SrC03 and BaC03 disclosed that the lattice of these samples were not distorted by the doping. Doped oxides a r e not effective on SrC03 and BaC03, while they are quite effective on MgO and CaO which can incorporate the added cation in the lattice. Since the cations of Ba2+ and Sr2+ are too large to be replaced with the doped cations, they a r e separated out. SrC03 and BaC03 behave like supports for the dopants. DKUSSION W e have proposed a reaction mechanism [12,14]. First of all, hydrogen is abstracted from CH4 by an active oxygen on the catalyst surface giving methyl radicals in the gas phase. This methyl radicals react with each other to give C2 hydrocarbons in the gas phase or they react with oxygen to COX on the surf ace. A CHI i /CO*
Now, we have proposed the two important factors of controlling the performance of catalysts; chemical factor and physical factor 112,141. The chemical factor is a factor to increase the density of active centers which a r e caused by doping through the lattice distortion. This causes synergetic effect between the promoter and additives. This factor would determine the activity (step A) and partly the selectivity (step B). Because a high concentration of the methyl radical (high activity) causes high Cs selectivity
area I m2g-1
C2 compounds (C2H4 and C2H6) yield at 1023 K over various alkallne earth oxides catalysts as a function of the specific surface area White circle indicates sample without dopants. Numbers indicate mol% of the dopant element. Elements without numbers indicate 0.2 mol% for MgO system and 10 mol% for CaO, SrC03, and BaCO3, respectively.
due to the 2nd order kinetics of Cp formation. The physical factor is represented by the morphology, for example, specific surface area in this study. This reflects the ratio of the size of the space close to the catalyst against the surface area. If produced methyl radical have higher possibility to react another methyl radical in a wide space, we may get higher C2 selectivity. But if produced methyl radical reacts with a lot of oxygens on the surface with a high specific surface area, C2 selectivity may decrease. Thus, the physical factor determines the C2 selectivity at step B (141. Similar surface area effects have been reported for the pyrolysis of methane (181. C2 selectivity has a maximum when the density of the active site is medium 1191. This phenomena has also been explained by a similar mechanism D91.
MgO and CaO systems have strong interaction between the additives and host oxides, which cause lattice distortion and high activity enough to consume most of the oxygen at 1023 K. In this case, step B is more important than step A. The ratio of O2 consumption by CHI consumption depends on two rate, the ratio between step El and step E2. With high C2 selectivity, CH4 conversion is high with the same 02 conversion around 95% (see also Fig. 3). On the other hand, there is little interaction between the additives and carbonates for SrC03 and BaC03 systems, which have low activities. The activity was proportional to the surface a r e a Interestingly, these system is considered to give almost the same activity and selectivity irrespective of the kind of additives if the specific surface area would be the same. In this case, step A is more important than step B, and the activity can be increased by increasing the surface area. CONCLUSIONS C2 yields over various metal oxide doped alkaline earth systems at 1023 K are rearranged as a function of CHq conversion in Fig. 3. The MgO system and the CaO system have apparently the same tend, and they a r e most promoted by addition of alkali metal oxides (see also Fig. 2). However, CaO system gave higher C2 yield (almost near to 25%) than MgO system a t this temperature and Thus, CaO system (especially LI+-CaO) was proved a t lower temperature too. to be better catalysts than MgO system (especially Na+-MgO). The role of promoter on CaO was thought to be similar to MgO system which has been discussed in detail [l2]. Promoters are considered to activate CaO surface structure more than MgO surface or CaO itself might have more active oxygen than MgO. Important things for the the catalyst design are not only the chemical factor (formation of active site), but also physical factor (specific surface
10 20 30 CH4 conversion / %
Fig. 3 C2 compounds yield a t 1023 K over various doped alkaline earth oxides catalysts as a function of methane conversion: doped MgO, ; doped CaO, ; doped SrC03 ; doped BaC03.
area or surface morphology). Alkali doped CaO system is the best catalyst because of having good performance in the two factors. Alkaline earth oxide doped CaO catalysts also have higher C2 yield than those of MgO catalysts probably due to the pronounced chemical factor.
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