The formation of intermediates plays a key role in reactivity, selectivity and deactivation in heterogeneous catalytic processes. However, their observation and determination remain a significant challenge due to the lack of selective techniques of sufficient sensitivity to detect their low concentrations (< 10 µmol/g).[1,2] We will show how an approach combining 13C isotopic enrichment with multinuclear multidimensional NMR and, on selected occasions, efficient Dynamic Nuclear Polarisation (DNP) Magic Angle Spinning (MAS) NMR at a range of external magnetic fields using tailored radicals as polarising agents, allows the fast detection of intermediates formed during catalysis. The approach is demonstrated in carbocations chemistry for a range of zeolites with different topologies including in Mobil-type five (MFI, e.g. H-ZSM5),[3] Zeolite beta polymorph A (BEA, e.g. beta zeolite)[4] and chabazite (CHA, e.g.H-SSZ-13, H-SAPO-34).[5]
We use two dimensional 13C-13C through-bond correlations to establish the carbon-carbon connectivity and unambiguously derive 5- and 6-membered ring cyclic carbocation and methylnaphthalenium ions as intermediates in the methanol to hydrocarbons catalytic reaction. We also showed that these species could be different even in zeolites with identical CHA topology.[5] These highlight that different catalytic routes exist for the formation of both targeted hydrocarbon products and coke exist.
We employ both 29Si-13C and 27Al-13C through-space experiments to quantitatively locate the confined carbocations with respect to the multiple surface sites of the zeolites, demonstrating that these species have strong van der Waals interaction with the frameworks and that their accumulation in the channels leads to deactivation. These results enable understanding of deactivation pathways and open up opportunities for the design of catalysts with improved performances.
We also show that introducing hierarchical pores into zeolites to form micro-meso-macroporous zeolite frameworks is a promising way to dramatically improve the overall DNP efficiency by a factor of ~ 4 on this type of materials[4] and may be a general method that could be applicable to other porous solids and heterogeneous catalysis processes.
[1] Kazanskii, V. B. Acc. Chem. Res. 1991, 12, 379.
[2] Xu, S. et al. Angew. Chem. Int. Ed. 2013, 44, 11564.
[3]Xiao, D. et al. Chem. Sci. 2017, 8, 8309.
[4] Xiao, D. et al. Chem. Sci. 2018, 9, 8184.
[5] Xiao, D. et al. RSC Adv.2019, 9, 12415.
Biography
Frédéric Blanc graduated from the University of Lyon (France) with a PhD in Chemistry in 2008 where he built structure – activity – dynamics relationship in heterogeneous catalysis and developed new solid state NMR methods to understand the chemistry of catalysts on surfaces. He then carried out postdoctoral work at the State University of New York (funded by the French Foreign Office) at the University of Cambridge (funded by the EU Marie Curie Fellowship scheme) understanding disorder and dynamics in energy materials from NMR spectroscopy. In December 2012, he was appointed to a Lectureship in the Department of Chemistry and the Stephenson Institute for Renewable Energy at the University of Liverpool and was promoted to a Personal Chair in October 2021. His current research interests focus on developing a broad range of advanced solid state NMR spectroscopy capabilities in materials chemistry and heterogeneous catalysis. Recent work includes enhancing the NMR signal of unreceptive nuclei (such as 17O at natural abundance) and capturing heterogeneous catalysts in the act. He leads the Connect NMR UK network that aims at maximising NMR usage in the UK and has a position on the Facility Executive of the UK High-Field Solid-State NMR National Research Facility providing access to the most advanced NMR infrastructures in the UK.