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| Author(s): |
Larissa de Bessa Lopes
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | São Carlos. |
| Institution: | Universidade de São Paulo (USP). Instituto de Química de São Carlos (IQSC/BT) |
| Defense date: | 2022-01-13 |
| Examining board members: |
Elisabete Moreira Assaf;
Fabio Bellot Noronha;
Alcineia Conceição Oliveira;
Fatima Maria Zanon Zotin
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| Advisor: | Elisabete Moreira Assaf |
| Abstract | |
The use of methane via oxidative coupling (OCM) is a promising process for obtaining C2 hydrocarbons. However, finding selective catalysts in OCM reactional conditions has been a challenge. Perovskite oxides present desirable characteristics for this reaction, such as high thermal stability and high basicity, which can be potentiated by partially replacing lattice cations with lower valence ones. Thus, the La1-yAyTi1-xMgxO3+δ (catalysts (being A = Ce or Sr; y = 0, 0.05 or 0.1 and x = 0, 0.25 or 0.5) were prepared and characterized to correlate the effects of the partial substitutions to their activities in the OCM reaction. Vibrational modes shift allowed studying the influence of substituting cations for the bond force weakening between metal and oxygen (M-O) in the catalyst lattice, and the contribution followed the order Mg > Sr > Ce. These effects are dependent on the polarizing power of each substituting cation, which alters the ionic bond character and, consequently, the catalysts basicity. LS10TM50O catalyst (La0,9Sr0,1Ti0,5Mg0,5O3+δ) presented the best OCM performance with 62,7 molsC2.kgcat-1h-1 C2 productivity due to the increased basicity, reducibility, and oxygen mobility of this catalyst compared to the others. Besides, the Mg and Sr enrichment on the surface, the increased formation of oxygen vacancies, and the lower Ti3+ species formation seem to be related to the most promising catalytic results. Optimizations in OCM conditions were performed using LS10TM50O as catalyst (complete factorial planning 23 with central point), aiming at better C2 selectivity. The temperature was the most influential variable, with an antagonistic effect, followed by the percentage of N2O e CH4/Oxid, both with synergistic effects. The best condition found was CH4/Oxid = 4, N2O = 100%, and T = 700 °C, obtaining 71.4% C2 selectivity, and was reproduced in a stability test (12 h) to evaluate catalyst performance over time. LS10TM50O catalyst remained stable, and C2 selectivity decreased from 73.3% to 60.7% at the end of the 12 h reaction. Post-reaction characterizations indicate that the selectivity decrease overtime may be correlated with changes in surface composition. In particular, due to the increase of Ti3+ species formation, which may have generated less basic active sites, more susceptible to overoxidation of the reaction products. Thus, the partial substitution of lattice cation with Mg and Sr showed synergistic effects to increase the selectivity of the reaction, and changes in the reaction conditions were fundamental to obtain even more promising results. (AU) | |
| FAPESP's process: | 17/03623-0 - Perovskite applied to the oxidative coupling of methane generated in the treatment of wastewater |
| Grantee: | Larissa de Bessa Lopes |
| Support Opportunities: | Scholarships in Brazil - Doctorate (Direct) |