J. CHEM. SOC. FARADAY TRANS., 1994, 90(21), 3357-3365 Oxidative Coupling of Methane over La,O, Influence of Catalyst Preparation on Surface Properties and Steady and Oscillating Reaction Behaviour V. R. Choudhary" and V. H. Rane Chemical Engineering Division, National Chemical Laboratory, Pune 4 1 1 008, India The catalyst precursor and calcination conditions used in the preparation of La,O, have been found to have a marked influence on the surface properties, basicity and base strength distribution, and catalytic activity and selectivity in the oxidative coupling of methane (OCM) under different reaction conditions. Among the La,O, catalysts, only that prepared from lanthanum acetate by decomposition in N, (but not in 0,)showed symmetric temperature and concentration oscillations in OCM at or below 933 K over a narrow temperature range.The unsteady reaction behaviour is found to be very complex and strongly dependent upon the OCM process param- eters and the catalyst parameters. Furthermore, the results on the La,O, obtained from the acetate indicated a strong possibility of the formation of new catalytic sites active at lower temperatures (below 873 K) during the OCM reaction at higher temperatures. Earlier studies'-7 showed that La203 has high activity and by drying at 393 K for 12 h. The lanthanum acetate (Aldrich) selectivity in OCM to C, hydrocarbons. La203 catalysts and lanthanum nitrate (GR, Loba) were ground with deion- with promoters such as Li,8 LiC1,' Na, , Sr and BaI8 ized water sufficient to form a thick paste and dried at 393 K have been studied as well as La-promoted MgO and for 12 h.The lanthanum hydroxide, lanthanum carbonate (I Ca0.'9-22 Taylor and Schrader6 reported that La203 and 11) were prepared by precipitation from an aqueous solu- obtained from different precursors showed different catalytic tion of ammonium hydroxide, sodium carbonate or ammon- performance in the OCM process. Recently we have observed ium carbonate, respectively, at pH 10-11 at room unsteady reaction behaviour, with periodic fluctuations in temperature. The precipitate was washed with deionized reaction temperature and concentration indicating symmetric water until free from cations and anions, and dried at 393 K oscillations, in OCM (above 823 K but below 973 K) over for 12 h.The dried catalyst mass was decomposed at 873 K La203 obtained from lanthanum acetate by thermal decom- for 6 h in static air, pressed binder-free and crushed to 22-30 position in N,.23We have now investigated the influence of mesh size particles, then calcined at 1023 and 1223 K in a precursor and reaction conditions on the bulk and surface flow of N, or 0, (12000 cm3 g-' h-' ). The calcination con- properties of La,03 and also on the catalytic activity and ditions of the catalysts are given in Table 1. selectivity and unsteady reaction behaviour in OCM under The surface area of the catalysts calcined at 1023 and 1223 different process conditions. K was determined by the single-point BET method by mea- suring the adsorption of nitrogen (30 mol% in He) at liquid- nitrogen temperature, using a Monosorb surface-areaExperimental analyser (Quanta Chrome Corp.).The crystal size and mor- The La,O, catalysts (Table 1) were prepared by thermal phology of the catalysts were studied by scanning electron decomposition of different precursors. The hydrated La,O, microscopy (SEM). The crystal phases were studied by was prepared by treating powdered La203 (Aldrich) with powder X-ray diffraction (XRD). deionized water (2 ml g-') on a water bath for 4 h while The acidity distribution on the catalysts was determined by maintaining a constant water content of the slurry, followed temperature-programmed desorption (TPD) of ammonia Table 1 Catalyst precursors and calcination conditions calcination conditions" surface area cat a1 y st precursor TIK atmosphere* /mz g-' Ia Ib IIa hydrated La203 hydrated La203 lanthanum acetate 1223 1023 1223 3.8 6.3 2.8 IIb lanthanum acetate 1023 4.5 IIC lanthanum acetate 1023 4.4 IIIa lanthanum nitrate 1223 1.7 IIIb lanthanum nitrate 1023 4.4 IVa IVb Va Vb VIa VIb lanthanum carbonate (I) lanthanum carbonate (I) lanthanum carbonate (11) lanthanum carbonate (11) lanthanum hydroxide lanthanum hydroxide 1223 1023 1223 1023 1223 1023 0.4 3.0 2.0 2.1 2.7 22.1 " Before calcination, catalyst precursor was decomposed at 873 K in static air for 6 h.The period of catalyst calcination was 2 h. Space velocity, 1200cm3 g-' h-'. (chemisorbed at 323 K) on the catalyst (0.5 g) from 323 to 1223 K at a linear heating rate of 20°C min-' in a flow of moisture-free helium (20 cm3 min- ') in a quartz reactor.The desorbed ammonia was detected with a thermal conductivity detector and also measured quantitatively by chemical analysis. The basicity and base-strength distribution on the catalysts were determined by measuring the step-wise thermal desorp- tion (STD) of CO, from the catalyst (0.5 g) in a quartz reactor, from 323 to 1173 K in a number of successive tem- perature steps (i.e. 323-423, 423-573, 573-773, 773-973 and 973-1 173 K). After the maximum temperature of the respec- tive step was attained, it was maintained for a period of 30 min to- allow desorption of the CO, adsorbed reversibly on the catalyst at that temperature. The amount of CO, desorbed in each step was determined gravimetrically by absorption in an aqueous barium hydroxide solution.The detailed procedure for measuring the base-strength distribu- tion by the STD of CO, and the estimation of CO, chemi-sorption data from the STD data have been described earlier.7*24 The CO, chemisorption data reported here are presented after subtracting the CO, content of the catalyst, which was determined by measuring the CO, evolved when the catalyst (after pretreatment at the calcination temperature in an He flow for 1 h) was heated from its calcination tem- perature to 1273 K in a flow of pure He for 1 h. Throughout, the chemisorption is considered as the amount of adsorbate retained by the presaturated catalyst after it was swept with pure He or N, for a period of 30 min.The steady/unsteady OCM reaction over the catalysts was carried out in a tubular quartz flow reactor packed with cata- lyst particles (0.1-0.2 g) between quartz wool plugs. The J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 reactor was kept in a vertical tubular furnace. The reaction temperature (controlled by a digital proportional tem-perature controller) was measured by a chromel-alumel thermocouple located in the catalyst bed. The temperature under unstcady conditions was measured as a function of reaction time. The reaction was carried out under following conditions: amount of catalyst, 0.1-0.2 g; feed, pure CH,-0, ; CH4/0,, 3.0-8.0; space velocity, 51 600-103 200 cm3 g- ' h-and temperature, i.e.reactor temperature, 673-1123 K. The product gases after the removal of water by conden- sation at 273 K were analysed by an on-line gas chromato- graph using Porapak-Q and Spherocarb columns. The concentration of 0, in the product stream was recorded con- tinuously by an on-line paramagnetic 0,-analyser (Oxymat I, Fuji Electric). High-purity gases He ( >99.99%), CH, (99.995%),CO, (99.995%),0, ( >99.5%)and NH, (99.99%)were used. Before being used the catalysts were pretreated in situ at their calcination temperature in a flow of He (20 cm3 min- l) for 1 h. Results and Discussion Surface Properties The surface areas of the catalysts are included in Table 1.The surface area of La,03 is strongly influenced by the precursors and the calcination temperature. The SEM micrographs of La,03 (IkVIb) are presented in Fig. 1 and that of La,03 (IIa) and La,03 (IIc) in Fig. 2. A comparison of the SEM photographs of La203 (I&-VIb) H1 prn Fig. 1 SEM micrographs of La,O, (a)Ib, (b)IIb, (c) IIIb, (d) IVb, (e)Vb and cf)VIb J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 (a1 H1 pm Fig. 2 SEM micrographs of La,O, obtained by calcination of La acetate (a) at 1023 K in 0, flow for 2 h and (b) at 1223 K in N, flow for 2 h shows a strong influence of the precursor on the crystal size and morphology. Furthermore, a comparison of the SEM micrographs of La203 (IIa, IIb and IIc) indicates that the crystal size and morphology of La203 (11) is influenced by the temperature and gas atmosphere used in the catalyst calcina- tion.XRD analysis of the La203 (I-VIa) catalysts obtained by the decomposition and calcination of the different catalyst precursors at 1223 K has indicated the presence of only pure La,03 crystalline phase. The La,03 (Ib, IIIb and VIb) cata-lysts were also found to contain only the La203 phase but the La203 (IIb, IVb and Vb) obtained from lanthanum acetate and lanthanum carbonate also showed the presence of minor amounts of La,(C03), and Laz0,C03 (lanthanum oxycarbonate). The XRD data have been presented else- where., The XRD results indicate no lanthanum carbonate or oxy- carbonate to be present in the catalysts calcined at 1223 K.However, these carbonates are not completely decomposed at 1023 K and hence are retained to a small extent in the bulk of the catalysts obtained from the decomposition of lantha- num acetate or carbonates. These observations are quite con- sistent with earlier st~dies.~~~~~~' The COz content of IIb, IVb and Vb was found to be 0.56, 0.75 and 1.35 mmol g-', respectively. The COz content of Ib, IIIb and VIb was found to be negligibly small. Ia-VIa showed no evolution of COz when heated up to 1373 K. The values of basicity (STD and chemisorption of CO,) for these catalysts are reported in this work after subtracting the values of their COz content. Fig. 3 shows the TPD curves of NH, from the catalysts, from their initial surface coverage by ammonia (ei) which cor- responds to the total acidity.It is seem that the acidity (measured in terms of NH, che-misorbed at 373 K) of La203 is strongly influenced by the precursor used in the catalyst preparation. The TPD curves (Fig. 3) show that the acid strength distribution on La203 is also very strongly influenced by the catalyst precursor. The presence of more than one peak in the TPD for all the cata- lysts reveals that there is more than one type of site for NH, 323 523 723 923 1123 1323 TI Fig. 3 TPD of ammonia on (a)Ia, Bi = 1.69 mmol g-'; (b) IIa, Bi = 0.69 mmol g- ;(c) IIIa, Bi = 0.24 mmol g-;(d)IVa, Bi = 0.23 mmol g-'; (e) Va, Bi = 0.18 mmol g-'; (f) VIa, Bi = 0.17 mmol g-I chemisorption. The ammonia chemisorption site (i.e.acid site) on the catalyst is expected to be a surface La3+. Thus, the results indicate that surface La3 + in different coordi- nations (3, 4 and 5 coordination) are present on the catalysts and their relative concentration is strongly influenced by the precursor. It is interesting to note that Ia has the highest acidity and contains only strong (major) and very strong (minor) acid sites compared with the other catalysts that are less acidic and also differ in their total acidity and acid strength dis- tribution. The temperature dependence of CO, chemisorption on the La,03 catalysts calcined at 1023 and 1223 K is shown in Fig. 4. The chemisorption of CO, at higher temperature points to the involvement of stronger basic sites.Hence, the CO, chemisorption us. temperature curves present the type of site energy distribution in which the number of sites are expressed in terms of the amount of CO, chemisorbed as a function of chemisorption temperature. The results (Fig. 4) indicate that the basicity distribution on the catalysts is very broad and is strongly influenced by both the precursor and the calcination temperature. The observed base-strength distribution on the catalysts is expected to be due to the presence of surface 0,-sites (i.e. basic sites) in different coordinations (3, 4 and coordination). The strong basicity is attributed to the low- coordinated 0,-surface sites. The total concentration of basic sites (measured in terms of CO, chemisorbed at 323 K) is decreased on increasing the calcination temperature.This is expected to be mostly due to decrease in the surface area by catalyst sintering. Whereas, the larger decrease in the strong basic sites (measured in terms of C02 chemisorbed at 773 K), relative to that of the total basic sites, on increasing the calcination temperature (Table 2) is mostly due to the removal of crystal defects which results in a decrease in low-coordinated surface 0,- J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1 I I I I I I using very high space velocity (103 200 cm3 g-' h-l). The (a1 results showing the influence of temperature on the methane 0.8 -conversion, C, selectivity, C2HJC,H, and CO/CO2product ratios are presented in Fig. 5-7.The following general obser- vations have been made : (1) For all the catalysts the ethene/ethane ratio is increased with increasing temperature. This is consistent with earlier studies.7.2 5.28-3 1 (2) The temperature dependence of the CO/CO, ratio in the products is different for different catalysts. The CO/CO2 ratio is decreased for IIa and IV-VIa but passes through a minimum or maximum for Ia and IIIa, depending upon the CH4/02ratio, with increasing temperature. (3) For IIa and IV-VIa the selectivity is increased with increasing temperature, the increase being very large for IVa. However, for Ia and IIIa the selectivity is decreased for CH4/02 = 4.0 but increased for CH4/02 = 8.0 with increas- ing temperature. (4) In general, the conversion is increased with increasing temperature.The increase is, however, very small for I-IIIa and Va at CH4/02 = 4.0. I I I I (b1 30r1 0 373 473 573 673 773 873 973 T/K Fig. 4 Temperature dependence of chemisorption of CO, on (a)b catalysts and (b)a catalysts. 0,I; 0,11; A, 111; V,IV; A, V and V, VI. The observed changes in the surface acidity and basicity with changes in the preparation conditions are due to changes in the coordination number of surface La3 + and 02-and probably also to the modification of the habits of the microcrystals produced with different crystal surface plane abundances. I 1 I I 973 1023 1073 1123 973 1023 1073 11: Steady OCM T/K T/K The OCM over I-VIa was carried out at 973-1123 K and Fig.5 Temperature dependence of (a)methane conversion and (b) Ia; 0,IIa; A, IIIa; A, IVa; 0,Va and V,VIawith a CH4/02 ratio of 4.0 and 8.0 at atmospheric pressure C, selectivity of 0, Table 2 Surface properties and catalytic activity and selectivity in OCM at 1023 and 1223 K, CHJO, = 4.0 and space velocity 103 200 Cm3 g-'h-' basicity/mmol g -' catalytic properties surface area acidity" CO, content catalyst /m2 g-' /mmol g-' /mmol g-' totalb strong' CH, conversion C, selectivity C, yield C,HJC,H, calcined at 1023 K I 6.3 0.00 0.346 0.227 24.0 55.5 13.3 1.03 I1 4.5 0.56 0.428 0.371 26.5 41.7 9.5 1.15 Ill 4.4 0.00 0.161 0.063 23.7 54.1 12.8 0.85 IV 3.0 0.75 0.602 0.567 22.2 55.2 12.3 1.60 V 2.1 1.35 0.710 0.640 23.7 51.4 12.2 1.oo VI 22.1 0.00 0.643 0.463 25.3 53.0 13.4 0.98 calcined at 1223 K I 3.8 1.69 0.00 0.091 0.056 25.0 57.0 14.3 1.11 I1 6.3 0.69 0.00 0.205 0.040 24.8 51.5 12.8 0.90 111 1.7 0.24 0.00 0.091 0.019 26.2 56.8 14.9 0.85 rv 3.4 0.23 0.00 0.148 0.101 4.2 28.6 1.2 0.14 V 2.0 0.18 0.00 0.126 0.090 27.2 55.6 15.1 0.90 V1 2.7 0.17 0.00 0.126 0.074 25.6 54.7 14.0 0.86 Measured in terms of NH, chemisorbed at 373 K.Measured in terms of CO, chemisorbed at 323 K. 'Measured in terms of CO, chemi- sorbed at 773 K. J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 0.4t /d P 0" 0.8 FO 0.61 I O.10.2 Ol I I 1 I 923 973 1023 1073 1123 T/K Fig. 6 Temperature dependence of ethene/ethane ratio over 0,la; e,IIa; A, IIIa; A,IVa; V, Va and V,VIa.CH,/O, :(a)4 and (b) 8. The above observations reveal a strong influence of cata- lyst precursor on the catalytic activity and selectivity in the OCM process. The results (Fig. 5 and 6)also show that an increase in the CH4/02 ratio from 4.0 to 8.0 causes a decrease in the conver- sion and the ethene/ethane ratio but the selectivity is gener- 1.6-1.2--0.8 0.4-N I I I 1 923 973 1023 1073 1123 T/K Fig. 7 Temperature dependence of the CO/CO, product ratio over 0,Ia; 0,IIa; A, IIIa; A,IVa; V, Va and V,VIa. CHJO,: (a)4 and (b) 8. ally increased. The influence of the CH4/02 ratio is quite similar to that observed for OCM over rare-earth metal oxides,' Mg029 and several other catalyst^.^^.^' The results of OCM over I-VIb at 873-1023 K, CH4/02 ratio of 4.0 and flow of 103 200 cm3g-h-' (at 1 atm) are presented in Table 3.They show that, in OCM over catalysts calcined at 1023 K, the reaction temperature has a strong influence on the conversion and product selectivity. The methane conversion, C, selectivity and ethene/ethane ratio increase but the CO/CO2 ratio decreases with increasing reaction temperature. The increase in the ethene/ethane ratio with decreasing CH4/02 ratio is most probably due to the availability of 0, at higher concentration for the following gas-phase reactions involved in the formation of ethyl radicals and ethene from et hane.32,3 C,H6 + 0, +C,H,' + HO,' (1) C2H5' + 02 --* C2H4 + HO2' (2) C2H6+ HO,' -+ C,Hs' + H,O, (3) H,O, + Z -+ 20H' + Z (4) (where Z is a third body, e.g.water molecule) C2H,j + OH' +C2H5' + H,O (5) Ethane is expected to be formed by gas-phase coupling of methyl radicals. 34 The increase in the ethene/ethane ratio with increasing temperature is expected to be due to the decomposition of ethyl radicals formed in reactions (l), (3) and (5) and thermal cracking of ethane at the higher temperatures CZHs' +C2H4 + H' (6) CzH6 +C,H4 + H2 (7) It may also be due to the increase in the rate of the gas-phase reaction of ethyl radicals to form ethene [reaction (2)] and the oxidative dehydrogenation of ethane on the catalyst surface. The increase in the C, selectivity with increasing tem- perature is expected to be mostly due to a decrease in the formation of carbon oxides from methyl radicals by the fol- lowing gas-phase reaction.34 CH,' + 02+CH,O,' + +CO, CO, (8) The formation of methylperoxy radicals (CH302'), which leads to CO and CO,, is not favoured at higher temperature~~,~~and hence the C, selectivity is expected to increase with the temperature.However, the decrease in the selectivity at the higher temperatures in some cases (Fig. 5) is attributed mostly to the conversion of methyl radicals to CO and CO, on the catalyst surface. To a small extent, it may also be due to combustion of ethane and ethene in the gas phase and/or on the catalyst surface at the higher tem- peratures. Note that unsteady reaction behaviour showing oscil- lations has been observed for IIb in a narrow temperature range 853 < T/K < 933.However, no unsteady (i.e. oscillating) reaction over the other catalysts at 933-1023 K was observed. The unsteady/steady OCM over La,O, IIa, IIb and IIc catalysts at different reaction conditions is discussed later. Comparison of Catalyst Surface Properties, Activity and Selectivity in Steady OCM The catalysts (1-VI), calcined at 1023 and 1223 K are com- pared for their surface properties and catalytic activity and selectivity in the steady OCM at 1023 K in Table 2. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 3 OCM over La,O, catalyst calcined at 1023 K catalyst reaction temperature/K CH, conversion (%) C, selectivity (%) C2HJC2H6 CO/CO, Ib 116 873 923 973 1023 873 883-923 15.7 21.5 23.5 24.0 23.1 oscillations observed 29.7 47.0 53.6 55.5 49.3 0.37 0.71 0.92 1.03 0.83 0.56 0.39 0.33 0.25 0.29 IIIb 973 1023 873 923 973 24.9 26.5 11.2 14.7 18.9 46.3 47.6 36.5 38.3 46.3 0.89 0.98 0.56 0.56 0.66 0.31, 0.28 0.56 0.54 0.45 IVb 1023 873 923 973 1023 23.7 12.7 20.8 22.0 22.2 54.1 53.6 55.2 54.9 55.2 0.85 0.86 0.97 1.21 1.60 0.39 0.50 0.43 0.37 0.33 Vb 873 923 973 6.9 22.7 22.2 10.0 42.7 48.5 0.63 0.71 <0.1 0.74 0.45 0.40 1023 23.7 51.4 1.oo 0.39 873 17.0 38.9 0.70 0.47 923 19.8 39.8 0.72 0.37 973 22.3 46.2 0.73 0.36 1023 25.3 53.0 0.98 0.29 ~~ Reaction conditions: amount of catalyst = 0.1 g, CHJO, ratio = 4.0 and space velocity (at STP) = 103 200 an3g-'h-'.The very low surface area and catalytic activity and selec- tivity of IVa (Table 2) may be due to sintering of the catalyst, probably related to the presence of traces of sodium in the La carbonate (I) obtained by precipitation with sodium carbon- ate.The increase in the catalyst calcination temperature from 1023 to 1223 K resulted in a large decrease in the surface area and basicity (both total and strong basicity) but, except for IV, a significant increase in both the methane conversion activity and selectivity, and also caused a change in the cata- lyst order for their surface and catalytic properties. The above comparison and the results in Fig. 1-7 and Tables 1-3 reveal a strong influence of the catalyst prep- aration conditions on the surface properties and catalytic activity and selectivity, in steady OCM.From the comparison of the catalysts for their surface acidity and/or basicity with that for the catalytic activity, C, selectivity or C, yield, it can be noted that there is no direct relationship between the surface acidity/basicity and the catalytic activity or selec-tivity, as observed for rare-earth oxides.' Also, in our earlier studies on the OCM over alkali- or rare-earth-metal pro- moted MgO and Ca02593' and alkali- or alkaline-earth-metal promoted rare-earth metal oxides,,' no direct relation- ship between the surface and catalytic properties was observed. The overall OCM process is very complex. It involves a number of catalytic (i.e. surface-catalysed) and non-catalytic (homogeneous or surface-initiated homo-geneous) reactions occurring sim~ltaneously.~~-~~ The con- tribution of the homogeneous reactions to the observed conversion and selectivity in the OCM process is quite appre- ciable and hence it is difficult to obtain a direct relationship between the surface properties and the catalytic activity and selectivity.Unsteady Reaction Behaviour The results indicate that only 11, prepared from La acetate, shows unsteady reaction behaviour in OCM over a narrow temperature range. In the earlier st~dies'-'*~~-~' oscillations were not observed. In our earlier paper,23 the results showing periodic fluctua- tions in reaction temperature and 0, concentration (in the product) indicate symmetric oscillations in OCM over IIb above 823 K but below 973 K.Additional results showing the influence of process parameters and catalyst parameters on the oscillatory behaviour in the OCM process over I1 are presented in Tables 4-6. The minimum and maximum of the 0, concentration oscillation correspond to the maximum and minimum, respectively, of the temperature oscillation. Note that the results of OCM under the unsteady conditions are indicative of changes in the conversion and product selec- tivity measured at close to the minimum and maximum of the oscillating reaction temperatures. It is extremely difficult to obtain results exactly at the minimum and maximum of the oscillating temperature. Efect of Process Parameters The influence of CH4/02 ratio (in the feed) on the steady/ unsteady reaction behaviour for the OCM process over I1 is shown in Table 4.The temperature and 0, concentration oscillations are observed only over a narrow temperature range, depending upon the CH4/02 ratio. For ratios 3.0, 4.0 and 5.0, the oscillations are observed only at 843 < T/ K < 943, 863 < T/K < 933 K and 873 < T/K < 903, respec-tively; for CH,/02 2 6.0, no oscillations were observed. Both the observed temperature and 0,concentration oscillations were symmetrical and sustained for a long period without affecting their amplitude Lie. T,,, -Tmin and C02(max) -COz(minJand cycle period. The recorded oscillations were given el~ewhere.~~.~~ In general, the cycle period and ampli- tude of both the oscillations are found to decrease with increasing reactor temperature.A comparison of the results in Table 5 with that in Table 4 for a CHJO, ratio of 4.0 show a high dependence on the amount of catalyst on the reaction behaviour. When the amount of catalyst in the reactor is changed from 0.1 g (Table 4) to 0.2 g (Table 5), then, at the same space velocity (103 200 cm3g-' h-') oscillations are observed at 883,903 and 923 K only for the smaller amount of catalyst. This indicates that when the linear or superficial gas velocity is doubled, the J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 4 OCM over I1 under steady and unsteady conditions at different CHJO, ratios ~ ~~~ CHJO, reactor temperaturew oscillatingtemperature/K oscillating 0, concentration in product (molyo) methane conversion (%) ~ C, selectivity C,H&,H, ~ CO/CO, IIb 3.0 853 800 f 5 (min) 18.0 f0.5 (max) 16.3 27.2 0.95 0.32 1005 f6 (max) 5.5 f0.4 (min) 28.6 43.0 1.06 0.10 873 838 f3 (min) 15.3 f0.2 (max) 18.9 29.1 0.83 0.72 1003 f 2 (max) 5.6 f0.2 (min) 22.4 42.6 1.01 0.20 923 924 f2 (min) 8.7 f0.2 (max) 22.4 33.1 0.76 0.25 964 f5 (max) 7.2 f0.2 (min) 23.4 37.6 0.86 0.33 973" 973 f5 6.2 f0.1 24.4 40.9 0.86 0.23 1023" 1023 f2 4.7 f0.1 28.6 44.0 1.20 0.15 4.0 883 885 f2 (min) 10.2 f0.1 (max) 23.2 48.8 0.88 0.20 989 f6 (max) 5.2 f 0.3 (min) 24.0 48.3 0.85 0.25 903 913 f2 (min) 8.8 f0.2 (max) 22.5 47.0 0.82 0.23 975 k4 (max) 6.0 f0.2 (min) 22.0 43.8 0.83 0.30 923 933 f3 (min) 7.7 f0.1 (max) 20.8 42.2 0.80 0.35 971 f6 (max) 6.2 f0.1 (min) 22.2 44.9 0.82 0.26 973" 973 f2 6.0 f0.2 24.9 46.3 0.89 0.31 1023" 1023f3 4.9 f 0.1 26.5 47.6 0.98 0.28 5.0 883 868 f2 (min) 13.1 f0.2 (max) 5.9 16.0 0.2 1 0.70 931 f5 (max) 9.7 f 0.3 (min) 7.7 30.0 0.27 0.28 923" 923 f5 9.2 f0.2 10.0 30.4 0.31 0.50 973" 973 f 3 6.5 f 0.1 12.0 30.8 0.35 0.77 1023" 1023 f2 4.9 f0.3 19.1 50.4 0.69 0.26 6.0 883" 883 f2 11.5 f 0.1 3.7 5.7 -0.9 1 923" 923 f 3 8.5 f0.1 8.5 25.8 0.2 1 0.59 973" 973 f 5 6.0 f0.2 14.4 42.8 0.43 0.41 1023" 1023 & 2 4.9 f0.1 16.8 51.3 0.60 0.28 IIC 4.0 873" 873 f 2 2.6 f0.1 23.1 49.3 0.83 0.29 923" 923 f 3 2.2 f0.1 23.7 48.9 0.94 0.29 973" 973 f1 1.9 f 0.3 23.5 46.9 1.02 0.33 1023" 1023 f1 1.8 f0.1 22.9 41.7 1.15 0.39 IIU 4.0 873" 873 f 2 No reaction 923" 889 f 2 (min) 5.2 14.1 0.15 0.50 996 f 3 (max) 18.2 46.7 0.74 0.27 973" 973 f3 23.9 48.0 0.82 0.23 1073" 1073 & 4 24.5 53.3 0.96 0.17 Reaction conditions; amount of catalyst, 0.1 g; feed, CH,-O, mixture of pure CH, and 0, and space velocity 103200 cm3 g-' h-I." Oscillations not observed. Table 5 OCM over IIb under steady and unsteady conditions at different space velocities oscillating 0, run reactor oscillating concentration in methane no. temperaturew tempe!rature/K products (molY0) conversion (%) C, selectivity (%) C,H&,H, CO/CO, ~ space velocity 51 600 cm' g-' h-' 823 823 f 2 no reaction 883" 883 f2 3.5 f0.1 26.4 46.7 0.82 0.22 903" 903f5 3.0 f0.2 28.8 46.9 0.87 0.27 923" 923 f 3 2.8 f0.3 27.4 49.9 0.9 1 0.16 973" 973 f3 2.5 f0.1 28.6 49.0 0.99 0.26 1023" 1023 f 5 2.9 f 0.1 28.2 50.7 1.05 0.17 823 782 f 1 (min) 9.2 f0.2 (max) 18.5 42.5 0.82 0.40 925 f5 (max) 3.0 f0.1 (min) 27.6 41.3 0.89 0.44 8 773 773 f1 20.0 f0.1 no reaction space velocity 103 200 cm3 g-' h-' 823 823 f 2 no reaction 873" 873 f5 7.5 f0.2 19.6 46.1 0.96 0.40 923" 923 f3 4.0 f0.1 24.0 43.1 1.21 0.54 973" 973 f3 3.5 f 0.3 23.7 41.3 1.35 0.55 1023' 1023 f 2 3.0 f 0.2 23.2 40.2 1.52 0.55 833" 833 f3 12.5 f0.3 11.1 47.8 0.78 0.2 1 808 784 f 3 (min) 16.0 f0.2 (max) 5.8 32.8 0.56 0.49 878 f 5 (max) 11.5 f0.1 (min) 13.3 45.3 0.62 0.42 8 773 773 f 1 20.0 & 0.1 no reaction Reaction conditions: amount of catalyst, 0.2 g and CHJO, ratio in feed, 4.0." Oscillations not observed. 3364 J. CHEM. SOC. FARADAY TRANS.,1994, VOL. 90 Table6 OCM over IIb with particle size of 30-72 mesh, under steady and unsteady conditions ~ run no. reactor temperaturew oscillating temperaturew oscillating 0, concentration in products (mol%) methane conversion (%) C, selectivity (%) C,HJC,H, CO/CO, 823 823 & 2 no reaction 873" 873 f2 3.8 f0.2 26.0 44.9 923" 923 f 5 3.5 f0.1 26.7 45.8 973" 973 f3 3.2 f0.2 28.1 47.0 1023' 1023 f 5 3.1 f0.1 28.3 47.7 833" 833 f2 6.8 f0.2 23.8 45.3 773" 773 f 5 9.0 f0.1 20.8 42.5 723 673 715 f8 (min) 765 f6 (ma) 673 f 1 16.7 f0.1 (max) 12.5 f0.2 (min) 20.0 f 1 13.3 15.1 no reaction 40.1 40.7 Reaction conditions: amount of catalyst,0.1 g, CHJO, ratio in fed, 4.0 and space velocity 103 200 cm3g-1.01 0.28 1.08 0.27 1.05 0.27 1.18 0.26 0.93 0.30 0.85 0.36 0.77 0.36 0.78 0.39 h-l." Oscillations not observed. unsteady reaction behaviour changes drastically. It is also interesting to note from the results in Table 5 that, for both space velocities the catalyst do not show any activity initially at or below 823 K but after the reaction at higher tem- peratures, it shows activity with oscillations at 823 K for a space velocity 51 600 cm3 8-l h-' and at 808 K for velocity 103 200 cm3 g-h-'. These observations indicates a possi- bility of formation of active sites responsible for the unsteady behaviour during the reaction at the higher temperatures.The increase in the space velocity resulted in a decrease in the temperature at which the oscillations occurred and also a decrease in the amplitude of both the oscillations. Efect of Catalyst Parameters A comparison of the results on IIa, IIb and IIc for CH,/02 = 4.0 (Table 4) shows that the catalyst calcination conditions (temperature and gas atmosphere, i.e. N, or 0,) have a strong influence on the unsteady reaction behaviour and also on the catalytic properties in steady OCM. In the OCM over IIc, no oscillations were observed and the C, selectivity was found to decrease with increasing reac- tion temperature. With IIa and IIb, oscillations are observed and the C2 selectivity is found to increase with increasing reaction temperature.These observations reveal a strong influence of the gas atmosphere used in the catalyst calcina- tion on the reaction behaviour and selectivity in the OCM process. The results on IIa and IIb (for CHJO, = 4.0) indicate that, when the catalyst calcination temperature is increased from 1023 to 1223 K, no reaction occurs at 873 K and the oscil-lations are observed only at 923 K, thus narrowing drasti- cally the temperature range for the unsteady reaction. A comparison of the results in Tables 4 and 6 shows that for IIb (at CHJO, = 4.0) when the particle size of the cata- lyst is changed from 22-30 mesh to 30-72 mesh, no unsteady reaction behaviour in the OCM was observed at 873-923 K but it was observed at 723 K after the reaction at higher temperatures.The results in Table 6 also reveal that, in this case, no catalytic activity was observed at 823 K but the cata- lyst showed activity at lower temperature after the reaction at higher temperatures, indicating the creation of new catalytic active sites on the catalyst during the OCM reaction at the higher temperatures. The above observations reveal that the unsteady reaction behaviour in the OCM process over I1 is very complex and strongly influenmd by both the process and catalyst param- eters. Aim, new sites, active at lower temperatures, are fornied on the catalyst during the OCM at higher tem- peratures. However, the nature of these sites has not been identified; further work is necessary for this.The observed unsteady reaction behaviour is likely to be due to a large difference in the C2 selectivity of the La203 catalyst in the OCM process at lower (<973 K) and higher (>973 K) temperatures (Table 4); the selectivity is decreased with decreasing temperature. In the pulse reaction of methane over nit-in presence of free 0,(CHJO, = 2.9), the C, selectivity at lower temperatures (823-923 K) was found to be much smaller (almost negligibly small) than at higher temperatures ( 2973 K).,' With IIc, the selectivity decreases with increasing temperature (Table 4) and, therefore, no oscil- lations are observed. In addition to the above the unsteady- state reaction behaviour could also be due to a change in the nature of the active sites on the catalyst.There is a possibility of the formation of surface La oxycarbonate (which is stable at lower temperatures) during the reaction at higher tem- peratures. Since, the unsteady reaction behaviour is observed only in the case of a particular catalyst sample, the tem- perature controller is not expected to have a significant role in the observed oscillations. Conclusions The following important conclusions have been drawn from the present investigation on the surface properties, the cata- lytic activity and selectivity, and the steady or unsteady reac- tion behaviour in OCM of La203 catalysts prepared using different catalyst precursors and calcination conditions : (1) The surface properties (uiz.surface area, crystal size and morphology, acidity and acid-strength distribution, basicity and base-strength distribution) and the catalytic activity and selectivity in OCM are strongly influenced by the catalyst precursor [uiz. hydrated La203, La nitrate, La acetate, La carbonate and La hydroxide]. The calcination temperature also has a strong influence on the surface and catalytic properties of La203 obtained from the different precursors. There is no direct relationship between the surface acidity/ basicity of La203 and its catalytic activity and selectivity in OCM. (2) Among the La203 catalysts, only the one obtained from La acetate and calcined in the presence of N, (but not in 0,) shows unsteady reaction behaviour with symmetrical tem- perature and concentration oscillations in the OCM process at below 933 K in a narrow temperature range.(3) The unsteady reaction behaviour in OCM on the La203 catalyst obtained from La acetate is very complex and strongly influenced by both the process parameters (uiz. tem-perature, CHJO, ratio in feed, space velocity and linear or superficial gas velocity) and catalyst parameters (uiz. particle size and calcination temperature and gas atmosphere used in the catalyst calcination). There is a strong possibility of for- J. CHEM. SOC. 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