Applied Catalysis, 53 (1989) L19-L21 Elsevier Science Publishers B.V.. Amsterdam -
L19 Printed in The Netherlands
Oxidative Methylation of Toluene with Methane over Alkali Metal Bromide Loaded Rare Earth Oxides TOSHIMITSU SUZUKI, KENJI WADA and YOSHIHISA WATANABE* Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyoto University, Sakyo-Ku, Kyoto 606 (Japan) (Received 16 June 1989)
ABSTRACT Oxidative methylation of toluene with methane over the NaBr loaded La,O, catalyst at 1023 K resulted in the production of styrene and ethylbenzene in a yield of 11.6% with a selectivity of 31.4%.
Methane is the most abundant component of natural gas which is used mainly as fuel. Direct utilization of methane as a chemical feedstock has attracted much attention in the past [ 1,2]. In addition to oxidative coupling of methane, oxidative methylation of toluene with methane to styrene and ethylbenzene has been investigated by Khcheyan et al. [ 31. Oxides of Bi, MO and Zn along with alkali and alkaline earth metals were reported to be effective catalysts. However, details of the catalysts have not been disclosed. In this communication, we report on novel rare earth oxides such as NaBr loaded La203, active and selective for oxidative coupling of toluene and methane. Porous rare earth oxide catalysts (further designated as La203-OX) were prepared by thermal decomposition of their oxalates at 1023 K. Loaded catalysts were prepared by impregnating aqueous solutions of alkali metal halides onto La,O,-OX described above. These catalysts were activated in flowing air (20 ml/min) at 1023 K for 2 h before the reaction. Reactions were carried out using a conventional fixed bed reactor for 3 h. The typical experimental conditions were as follows; amount of catalyst =40 mg, temperature= 1023 K, methane flow-rate = 25 mmol/h, CH, : 0, : CO, (diluent ) : toluene = 50 : 5 : 45 : 3 (molar ratio ) . Toluene was introduced by passing the reactant gas mixture into a toluene vapor saturator just before the inlet of the reactor. Yield of CO, was
0 1989 Elsevier Science Publishers B.V.
L20 TABLE 1 Oxidative methylation of toluene over rare earth oxide at 1023 K Amount of catalyst= 40 mg, CH,:02:COs:toluene=50:5:45:3 Catalyst
Quartz wool La20sa La,O,-OX La,O,-OXd LiBr (5 wt.-% : Li)/La,O,-OX NaBr (5 wt.-% : Na)/La,O,-OX KBr (5 wt.-% : K) /La,O,-OX KBr (5 wt.-% : K)/La203-OX* NaBr (5 wt.-% : Na)/La,O,-OX’
W/F=O.EiO g- h/mol,
C&-Yield Selectivity ( W) (%) EthylStyrene Benzene Cs benzene 2.7 5.6 6.2 5.7 6.4 11.6 7.5 7.0 1.6
29.8 16.3 21.2 17.1 28.6 31.4 24.2 28.5 4.5
23.2 8.5 14.6 10.3 13.9 11.0 15.7 18.5 1.0
6.6 7.8 6.6 6.8 14.7 20.4 8.6 10.0 3.6
22.5 25.1 20.8 19.2’ 17.2 23.6 17.3 8.9 22.1
2.3 33.8 10.2 1.1 44.5 12.9 1.2 43.8 13.1 2.0 29.1 31.8’ 5.3 25.1 23.9 3.1 20.4 21.5 0.0 33.6 24.9 5.6 28.7 28.4 7.1 30.2 36.1
“Amount of catalyst = 100 mg, W/F= 2.0 g*h/mol. ‘Reaction temperature = 973 K. ‘Reaction without CH,, O2: CO2 : toluene = 5 : 95 : 3. dDiluent = helium. ‘Coke deposition was observed.
estimated from the balance between oxygen feed and the amount of oxygencontaining products and recovered oxygen. Various rare earth oxides were tested for activity and selectivity toward styrene and ethylbenzene (C&-compounds) under specific reaction conditions, and La203, which is known to be an effective catalyst for the oxidative dimerization of methane [ 41, was found to be active for the methylation of toluene. Table 1 shows the results of the reaction at 1023 K. The products were benzene, C8compounds, stilbene and 1,2diphenylethane (C,,-compounds), CO, COz, water and hydrogen along with ethane, ethene and a very small amount of unidentified hydrocarbons. As seen in Table 1, the effect of the quartz wool plug used for support of the catalyst bed was examined, and a 2.7% yield of C,-products was observed. This indicates that some cross coupling proceeded thermally. The best C8-yield with the commercial La,O, was 5.6%. With La,O,-OX, a higher C&-yield (6.2% ) and C&-selectivity (21.2% ) were obtained. Space time yield (S.T.Y.) for the C&-compounds with La,O,-OX (2.3 mmol/g*h) was higher than that with the commercial catalyst (0.9 mmol/g.h ) . When as a diluent for the reaction gas mixture helium was used instead of COB, both C8yield and C&-selectivity decreased and the amount of CO, formed increased. Coke deposition on the catalyst surface was observed after the reaction under
helium. This suggests that use of CO2 as a diluent for the reaction gas mixture is effective to retard complete oxidation of substrates and deposition of coke. The promoter effect of alkali metal bromide  in the La203-OX catalyst was studied, All bromides were loaded with 5 wt.-% of alkali metal to La,O,OX. The NaBr/La,O,-OX showed the highest Cs-yield of 11.6% with a selectivity of 31.4%. It is worth noting that even at lower reaction temperature, 973 K, high ($-yield (7.0% ) was obtained with KBr/La,O,-OX. No decreases in &-yields and selectivities were observed during the reaction time of 3 h with all alkali bromide loaded La203. Although a slight loss of alkali bromides from the catalyst bed during the reaction seems to occur, much larger amounts of C,-compounds were produced for 3 h than the amount of NaBr and La20, employed. Several probable reaction paths for oxidative methylation of toluene can be considered. The reaction of toluene without methane in the feed was carried out, but only small amounts of Cs-compounds were obtained (see Table 1). The reaction between toluene and ethane in place of methane gave n-propylbenzene as a main product. These results strongly suggest that the synthetic route to styrene and ethylbenzene from toluene involves the step of oxidative cross-coupling between methane and toluene, followed by dehydrogenation of the produced ethylbenzene.
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