A site-sensitive quasi-in situ strategy to characterize Mo/HZSM-5 during activation

Abstract

The active sites on the methane dehydroaromatization (MDA) catalyst Mo/HZSM-5 are very hard to characterize, because they are present in various geometries and sizes and only form under reaction conditions with methane at 700 ºC. To address these issues an experimental strategy is presented that enables distinguishing different active sites for MDA present on Mo/HZSM-5 and helps determining the Mo charge, nuclearity and chemical composition. This approach combines a CO pretreatment to separate the active Mo site formation from coke formation, quasi-in situ spectroscopic observations using DNP, 13 C NMR, CO IR and theory. This allows the discrimination between three different types of active sites. Distinct spectroscopic features were observed corresponding to two types of mono-or dimeric Mo (oxy-)carbide sites as well as a third site assigned to Mo2C nanoparticles on the outer surface of the zeolite. Their formal Mo oxidation state was found to be between 4+ and 6+. Dynamic nuclear polarization (DNP) measurements of samples carburized in CO as well as in CH4 confirm the assignment and also show that accumulated aromatic carbon covers the bigger Mo nanoparticles on the outer surface of the zeolite, causing deactivation. It was previously observed that after an initial period where no desired products are formed yet, benzene starts slowly forming until reaching its maximum productivity. Direct observation of the active site with 13 C NMR confirmed that Mo-sites do not transform further once benzene starts forming, meaning that they are fully activated during the period where no desired products are observed yet. Therefore the slow increase of the benzene formation rate cannot be attributed to a further transformation of Mo sites.

Activity Descriptors Derived from Comparison of Mo and Fe as Active Metal for Methane Conversion to Aromatics

“…3,13,[32][33] This transformation marks the 'activation period' of the catalyst as benzene is only produced after Mo (oxy-)carbides are formed (Scheme 1). [34][35] We found that this activation can also be achieved with a pre-treatment under CO atmosphere, which leads to an equivalent active phase of Mo. 36 During this treatment, no carbonaceous deposits are formed as would during activation in CH4.…”

Initial Carbon−Carbon Bond Formation during the Early Stages of Methane Dehydroaromatization

“…However, a strong debate (Section S2.1) about the structure of active Mo sites (MoO x C y , Mo x C y etc. ), location (pore surface vs. external surface) and their role in the reaction mechanism is still to be resolved . Besides the ongoing debates regarding the active sites, gaining insight into the initial C−C bond formation mechanism and further oligomerization–cyclization of the intermediates is critical.…”

Revival of Zeolite‐Templated Nanocarbon Materials: Recent Advances in Energy Storage and Conversion

“…Lewis acidity of zeolites has been widely used in heterogeneous acid catalysis. [128][129][130][131] The Lewis acidity originates from substituted Al (III) in the frameworks of zeolites. In the process of zeolite-templated methods, polycyclic aromatic compounds are liable to deposit on the acidic sites, [132] which has positive effect on the formation of nanocarbon structures inside the nanochannels of zeolites.…”