Title
A site-sensitive quasi-in situ strategy to characterize Mo/HZSM-5 during activation
Author
Vollmer, I. (TU Delft ChemE/Catalysis Engineering)
Kosinov, N. (Eindhoven University of Technology)
Szécsényi, Ágnes (Student TU Delft)
Li, G. (TU Delft ChemE/Catalysis Engineering) 
Yarulina, I. (King Abdullah University of Science and Technology)
Abou-Hamad, Edy (King Abdullah University of Science and Technology)
Gurinov, Andrei (King Abdullah University of Science and Technology)
Ould-Chikh, Samy (King Abdullah University of Science and Technology)
Aguilar-Tapia, Antonio (Université Grenoble Alpes)
Hazemann, Jean Louis (Université Grenoble Alpes)
Pidko, E.A. (TU Delft ChemE/Inorganic Systems Engineering; TU Delft ChemE/Algemeen)
Hensen, Emiel (Eindhoven University of Technology)
Kapteijn, F. (TU Delft ChemE/Catalysis Engineering)
Gascon, Jorge (TU Delft ChemE/Catalysis Engineering; King Abdullah University of Science and Technology)
Date
2019
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 °C. 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, 13C 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 13C 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.
Subject
C NMR
CO IR
Dimeric species
DNP SENS
Methane dehydroaromatization
Mo (oxy-)carbide
Mo/HZSM-5
MoC
Monomeric species
Well-defined species
To reference this document use:
http://resolver.tudelft.nl/uuid:975cbec0-1352-4b69-8a6a-088671da4042
DOI
https://doi.org/10.1016/j.jcat.2019.01.013
Embargo date
2021-01-22
ISSN
0021-9517
Source
Journal of Catalysis, 370, 321-331
Bibliographical note
Accepted Author Manuscript
Part of collection
Institutional Repository
Document type
journal article
Rights
© 2019 I. Vollmer, N. Kosinov, Ágnes Szécsényi, G. Li, I. Yarulina, Edy Abou-Hamad, Andrei Gurinov, Samy Ould-Chikh, Antonio Aguilar-Tapia, Jean Louis Hazemann, E.A. Pidko, Emiel Hensen, F. Kapteijn, Jorge Gascon