Non-oxidative methane dehydroaromatization on Mo/HZSM-5 catalysts: Tuning the acidic and catalytic properties through partial exchange of zeolite protons with alkali and alkaline-earth cations

• Published in: Applied catalysis. A, General General, 2016-04, Vol.515, p.32-44
• Main Authors: Martínez, Agustín, Peris, Ernest
• Format: Article
• Language: English
• Subjects: Alkali and alkaline earth cations; Benzene; Catalysis; Catalyst stability; Catalysts; Exchange; Exchanging; Methane; Methane dehydroaromatization; Mo/HZSM-5; Selectivity; Zeolite exchange; Zeolites
• ISSN: 0926-860X; 1873-3875
• DOI: 10.1016/j.apcata.2016.01.044

[Display omitted] •Partial exchange of H+ in HZSM-5 with Na+/Cs+ decreases coking and improves benzene selectivity and stability for MDA.•Partial exchange with Ca2+/Mg2+ inhibits migration of Mo within the zeolite channels and worsens MDA performance.•Framework “Al pairs” play a key role for optimizing the catalytic performance of Mo/HZSM-5 catalysts for MDA. In this work, partial exchange of protons in H-ZSM-5 zeolite with alkali (Na+, Cs+) and alkaline-earth (Ca2+, Mg2+) cations was applied as a simple means of modulating the density and strength of the zeolite Brønsted acid sites with the aim of depressing the coke-forming tendency and, hence, of improving the stability of Mo/ZSM-5 catalysts (3wt% Mo, Si/Al=15) for methane dehydroaromatization (MDA). The materials were characterized by ICP-OES, XRD, N2 physisorption, 27Al MAS NMR, H2-TPR, DRS UV–vis, XPS, FTIR-pyridine, and NH3-TPD. Coke in spent catalysts was characterized by TGA and TPO. The MDA experiments lasted about 8h and were performed at the standard conditions of 700°C, 1bar, and GHSV of 1500cm3/(gcath). The preferential neutralization of the most acidic OH groups (likely associated to “isolated Al” atoms in the zeolite framework) by Na+ reduced both the amount and the average strength of the remaining Brønsted acid sites. The reduction in the acid site density decreased the amount of (Mo2O5)2+ and/or (MoO2)2+ species anchored to the zeolite framework (precursors of the active MoCx/MoCxOy nanoclusters) and, consequently, the methane conversion rate. However, an optimum exchange level (corresponding to a Na/Al atomic ratio of 0.1) was found for which the derived catalyst (Mo/Na10HZ5) exhibited an unusually low coke selectivity of ca. 10% (on a C basis) as compared to the selectivity of ca. 36% obtained for the parent Mo/HZ5 catalyst. Concomitantly, the selectivity to benzene raised from 42% for Mo/HZ5 to 66% for Mo/Na10HZ5 and the decay rates for both methane conversion and benzene formation (at TOS>6h) decreased by about one order of magnitude for Mo/Na10HZ5 with respect to Mo/HZ5. The increased benzene selectivity and stability lead to higher and more sustained benzene formation rates during the last reaction stages (TOS>6h) for Mo/Na10HZ5 in spite of its lower activity for methane conversion in comparison to Mo/HZ5. Similar trends, though less pronounced, were observed for the catalyst comprising the zeolite exchanged with Cs+ at the optimum exchange level of M+/Al=0.1. On the other hand, th