Oxidative coupling of methane on Sr/La2O3 catalysts: Improving the catalytic performance using cordierite monoliths and ceramic foams as structured substrates

SOLLIER, BRENDA; MIRO, EDUARDO; BOIX, ALICIA V.; GOMEZ, LETICIA

APPLIED CATALYSIS A-GENERAL

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2016 vol. 532 p. 65 - 76

0926-860X

tThis work shows how the use of structured catalysts results in an important increase of the C2yield in theoxidative coupling of methane over Sr/La2O3catalysts. Solids with 2 and 5 wt.% of Sr (Sr2 and Sr5) wereprepared and washcoated on cordierite monoliths and alumina foams. Homogeneous and mechanicallystable catalytic films were obtained in both cases with the addition of silica as binder. Other types of foamswere also prepared (aluminum silicate and magnesia stabilized zirconia) but in such cases the adhesionof coatings was rather poor. The catalytic surface XPS characterization of both powders and structuredcatalysts indicates that lanthanum is mainly present as hydroxide and carbonate and that strontium ismainly present as carbonate. However, in the case of washcoated cordierite monoliths, a modification inLa 3d spectra measured by XPS suggests a strong interaction of La with its chemical environment, mostprobably with silica and/or magnesia. This interaction could be originated by migration of the cordieritecomponents towards the catalytic film, as seen by EDX. The decomposition of La(OH)3and La2O2CO3intoLa2O3under reaction conditions was confirmed by in situ LRS. While hydroxide prevails at low temper-ature, the presence of oxycarbonates starts to be relevant when the reaction takes place at temperaturesabove 400◦C and the oxycarbonates decompose into oxide over 700◦C. The monolithic catalysts (M)Sr2and (M)Sr5 resulted twofold more active towards ethane and ethylene than the corresponding Sr2 andSr5 powders. Meanwhile, the washcoated alumina foam (FAl)Sr5 showed methane conversion valuessimilar to those of the powder but higher C2selectivity; thus, the C2yield was also higher. In the case ofthe monolithic catalysts, the increase in methane conversion and C2yield could be related to the interac-tion of Sr and La with silica and/or magnesia present in the cordierite structure. In the case of the aluminafoam catalyst, the higher selectivity could be originated by the lower gas phase/catalytic surface ratio(as compared with the monolith), that inhibits gas-phase combustion of C2products. In order to assessthe stability of (M)Sr5, which was the best catalyst among the ones studied in this work, time-on streamruns at 600 and 800◦C were performed. Surprisingly, at 800◦C, the C2yield steeply increased from 18%to 22.5% during the first 70 h and then remained constant until 100 h. This phenomenon could be dueto a gradual increase in chemical interactions between the cordierite components and the catalytic coat.At lower temperature (600◦C), C2yield was constant during the 100 h of time-on-stream, indicating thatthis temperature is not enough to produce the said effect.