Understanding the entanglement between diffusion and reaction by probing the mobility of ketene in chabazites Wei Chen, Pieter Cnudde, Veronique Van Speybroeck Center for Molecular Modeling (CMM), Ghent University, Technologiepark-Zwijnaarde 46, 9052, Ghent, Belgium E-Mail:
[email protected] Website: https://molmod.ugent.be Background Ketene (CH₂=C=O) is a key reaction intermediate in zeolite catalysis, appearing in syngas conversion, CO₂ hydrogenation, methanol-to-hydrocarbons (MTH), and dimethyl ether (DME) carbonylation. Inside zeolite voids, reactive ketene can transform into high-value chemicals via pathways such as methylation– decarbonylation, alkene formation, methyl acetate production, and hydrogen transfer. Because slow ketene diffusion can lower turnover frequencies and promote pore blocking through polymerization, understanding its transport behavior is critical for optimizing catalytic performance. Motivation Brønsted acid sites (BAS) are known to lower alkene diffusion barriers in HSAPO-34 through π–H interactions, reducing ethene barriers from ~38 to ~20 kJ mol⁻¹ without inducing protonation. Ethene and ketene share similar size, but ketene is far more reactive, with protonation barriers below 20 kJ mol⁻¹. Thus, ketene may be readily protonated to acylium ions or surface acetates, or react with oxygenates (e.g., water, methanol) to form stable products. These transformations could either facilitate or hinder ketene transport, leaving the impact of BAS on ketene diffusion under operando conditions uncertain. Collective variable to describe diffusion -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 0 10 20 30 40 50 Free energy (kJ/mol) ξ (Å) -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 0.0 0.2 0.4 0.6 0.8 1.0 sinα ξ (Å) α -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 3.5 3.6 3.7 3.8 3.9 4.0 8R radius (Å) ξ (Å) 168° < α < 175° -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 -1.0 -0.5 0.0 0.5 1.0 cosβ ξ (Å) 0 8 17 25 33 42 50 58 67 Free energy (kJ/mol) 0° < β < 37° Deprojection of 1D to 2D free energy surface Classical 1D diffusion free energy surface 43.1 ± 2.8 Ketene diffusion accelerated by BAS Ketene diffusion impeded by water Gating effect of water Highlights ➢The diffusion and reaction of ketene in chabazite are intrinsically entangled. ➢Ketene can be protonated to acylium ion or surface acetate during diffusion. ➢The co-feeding of water, methanol, and DME modulated ketene diffusion differently. ➢2D free energy surfaces obtained by deprojection techniques can disentangle reaction and diffusion in zeolite. Conclusion (1) The diffusion barriers in H-SAPO-34 are generally lower than in H-SSZ-13 because of a higher window surface area in the flexible SAPO framework; (2) The interaction of BAS with ketene along the diffusion path allows either the formation of stabilizing π-H interactions, or the formation of acylium ions or surface acetate upon protonation by the BAS, both resulting in a substantial decrease in the diffusion barrier; (3) Water, methanol, and DME impede ketene diffusion in H-SSZ-13 to a different extent, correlating to their strong, preferred adsorption interactions with the BAS; (4) High methanol loadings will lead to beneficial diffusion paths, owing to the formation of a protonated methanol cluster with delocalization of the positive charge and facilitation of favorable hydrogen bonding interactions. Furthermore, the tight interactions of ketene with adsorbed water or methanol opens up favorable pathways for the formation of acetic acid or methyl acetate, respectively.
