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

Ina Vollmer, Nikolay Kosinov, Ágnes Szécsényi, Guanna Li, Irina Yarulina, Edy Abou-Hamad, Andrei Gurinov, Samy Ould-Chikh, Antonio Aguilar-Tapia, Jean Louis Hazemann, Evgeny Pidko, Emiel Hensen, Freek Kapteijn, Jorge Gascon (Corresponding author)

Research output: Contribution to journalArticleAcademicpeer-review

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.

LanguageEnglish
Pages321-331
Number of pages11
JournalJournal of Catalysis
Volume370
DOIs
StatePublished - 1 Feb 2019

Fingerprint

Methane
Carbon Monoxide
Benzene
Zeolites
Chemical activation
activation
Nuclear magnetic resonance
methane
Polarization
Nanoparticles
benzene
Coke
Carbides
Carbon
Productivity
nanoparticles
nuclear magnetic resonance
coke
polarization
Oxidation

Keywords

  • C NMR
  • CO IR
  • Dimeric species
  • DNP SENS
  • Methane dehydroaromatization
  • Mo (oxy-)carbide
  • Mo/HZSM-5
  • MoC
  • Monomeric species
  • Well-defined species

Cite this

Vollmer, I., Kosinov, N., Szécsényi, Á., Li, G., Yarulina, I., Abou-Hamad, E., ... Gascon, J. (2019). A site-sensitive quasi-in situ strategy to characterize Mo/HZSM-5 during activation. Journal of Catalysis, 370, 321-331. DOI: 10.1016/j.jcat.2019.01.013
Vollmer, Ina ; Kosinov, Nikolay ; Szécsényi, Ágnes ; Li, Guanna ; Yarulina, Irina ; Abou-Hamad, Edy ; Gurinov, Andrei ; Ould-Chikh, Samy ; Aguilar-Tapia, Antonio ; Hazemann, Jean Louis ; Pidko, Evgeny ; Hensen, Emiel ; Kapteijn, Freek ; Gascon, Jorge. / A site-sensitive quasi-in situ strategy to characterize Mo/HZSM-5 during activation. In: Journal of Catalysis. 2019 ; Vol. 370. pp. 321-331
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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.",
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Vollmer, I, Kosinov, N, Szécsényi, Á, Li, G, Yarulina, I, Abou-Hamad, E, Gurinov, A, Ould-Chikh, S, Aguilar-Tapia, A, Hazemann, JL, Pidko, E, Hensen, E, Kapteijn, F & Gascon, J 2019, 'A site-sensitive quasi-in situ strategy to characterize Mo/HZSM-5 during activation' Journal of Catalysis, vol. 370, pp. 321-331. DOI: 10.1016/j.jcat.2019.01.013

A site-sensitive quasi-in situ strategy to characterize Mo/HZSM-5 during activation. / Vollmer, Ina; Kosinov, Nikolay; Szécsényi, Ágnes; Li, Guanna; Yarulina, Irina; Abou-Hamad, Edy; Gurinov, Andrei; Ould-Chikh, Samy; Aguilar-Tapia, Antonio; Hazemann, Jean Louis; Pidko, Evgeny; Hensen, Emiel; Kapteijn, Freek; Gascon, Jorge (Corresponding author).

In: Journal of Catalysis, Vol. 370, 01.02.2019, p. 321-331.

Research output: Contribution to journalArticleAcademicpeer-review

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T1 - A site-sensitive quasi-in situ strategy to characterize Mo/HZSM-5 during activation

AU - Vollmer,Ina

AU - Kosinov,Nikolay

AU - Szécsényi,Ágnes

AU - Li,Guanna

AU - Yarulina,Irina

AU - Abou-Hamad,Edy

AU - Gurinov,Andrei

AU - Ould-Chikh,Samy

AU - Aguilar-Tapia,Antonio

AU - Hazemann,Jean Louis

AU - Pidko,Evgeny

AU - Hensen,Emiel

AU - Kapteijn,Freek

AU - Gascon,Jorge

PY - 2019/2/1

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N2 - 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.

AB - 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.

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KW - CO IR

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KW - Methane dehydroaromatization

KW - Mo (oxy-)carbide

KW - Mo/HZSM-5

KW - MoC

KW - Monomeric species

KW - Well-defined species

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Vollmer I, Kosinov N, Szécsényi Á, Li G, Yarulina I, Abou-Hamad E et al. A site-sensitive quasi-in situ strategy to characterize Mo/HZSM-5 during activation. Journal of Catalysis. 2019 Feb 1;370:321-331. Available from, DOI: 10.1016/j.jcat.2019.01.013