Selectivity descriptors for the direct hydrogenation of CO2 to hydrocarbons during zeolite-mediated bifunctional catalysis

Abstract

Cascade processes are gaining momentum in heterogeneous catalysis. The combination of several catalytic solids within one reactor has shown great promise for the one-step valorization of C1-feedstocks. The combination of metal-based catalysts and zeolites in the gas phase hydrogenation of CO2 leads to a large degree of product selectivity control, defined mainly by zeolites. However, a great deal of mechanistic understanding remains unclear: metal-based catalysts usually lead to complex product compositions that may result in unexpected zeolite reactivity. Here we present an in-depth multivariate analysis of the chemistry involved in eight different zeolite topologies when combined with a highly active Fe-based catalyst in the hydrogenation of CO2 to olefins, aromatics, and paraffins. Solid-state NMR spectroscopy and computational analysis demonstrate that the hybrid nature of the active zeolite catalyst and its preferred CO2-derived reaction intermediates (CO/ester/ketone/hydrocarbons, i.e., inorganic-organic supramolecular reactive centers), along with 10 MR-zeolite topology, act as descriptors governing the ultimate product selectivity.

First-Generation Organic Reaction Intermediates in Zeolite Chemistry and Catalysis

“…For a more fundamental understanding, we refer to the book chapter by McKenzie and Rounder on this topic . Because of its ability to explore the mobility aspect of the reaction intermediate, the TF-μSR technique could be combined complementarily with the mobility-dependent solid-state NMR techniques to explore the host–guest chemistry in zeolite catalysis. , …”

“…If it is a descriptor, then can it control the product selectivity? The research groups, including us, Bao, Hemberger, Plessow/Studt, and Liu/Zheng strongly favor ketenes as a reaction intermediate. ,,,,− However, a few other studies proposed it to be unstable and highly reactive, and ketenes could accelerate the formation of coke species, which could lead to catalyst deactivation and, hence, the formation of ketenes is detrimental in zeolite catalysis. However, we can counterargue that this logic applies to all intermediates in zeolite catalysis and not necessarily to ketene.…”

“…Although ketenes have not been detected directly, Ramirez et al also recently hypothesized that they play a crucial role during the hydrogenation of CO 2 over Fe 2 O 3 @KO 2 /zeolite catalysts (Figure ). To explain the zeolite-dependent selectivity preference during the reaction, they performed solid-state NMR spectroscopic investigation after the hydrogenation of 13 CO 2 . The postreacted zeolites demonstrated several carbonylated species in the 13 C– 13 C correlations spectra (Figure a), where the carbons were polarized through direct excitation, and 13 C– 13 C mixing was achieved through proton-driven spin-diffusion (PDSD): zeolite surface-acetate species (Θ, 176.6 ppm/19.2 ppm ( 13 C) on FER and 180.2/18.8 ppm in ZSM-22), acetone (Ω, 211.9 ppm/29.8 ppm), and diacetyl (∂, 197.5/23.05 ppm in ZSM-5, and ∂, 198.9/24.12 ppm in ZSM-22) .…”

“… To explain the zeolite-dependent selectivity preference during the reaction, they performed solid-state NMR spectroscopic investigation after the hydrogenation of 13 CO 2 . The postreacted zeolites demonstrated several carbonylated species in the 13 C– 13 C correlations spectra (Figure a), where the carbons were polarized through direct excitation, and 13 C– 13 C mixing was achieved through proton-driven spin-diffusion (PDSD): zeolite surface-acetate species (Θ, 176.6 ppm/19.2 ppm ( 13 C) on FER and 180.2/18.8 ppm in ZSM-22), acetone (Ω, 211.9 ppm/29.8 ppm), and diacetyl (∂, 197.5/23.05 ppm in ZSM-5, and ∂, 198.9/24.12 ppm in ZSM-22) . Specifically, the simultaneous existence of acetone, acetate, and diacetyl groups strongly suggests the involvement of ketene (CH 2 CO) as an active reaction intermediate.…”

“…Evidence of ketene’s indirect involvement as a reaction intermediate during hydrogenation of CO 2 over Fe 2 O 3 @KO 2 /zeolite bifunctional catalysts. (a) 13 C– 13 C correlations in the carbonyl regions on postreacted zeolites MOR (pink), FER (blue), ZSM-5 (yellow), and ZSM-22 (green), highlighting the presence of ester and ketones (*: spinning sidebands on ZSM-5) . (b) The proposed interconversion between ketene and other carbonylated intermediates (surface acetate, acetone, diacetyl) on zeolite .…”