First-Principles Study of n-Butane Monomolecular Cracking and Dehydrogenation on Two-Dimensional-Zeolite Model Systems: Reaction Mechanisms and Effects of Spatial Confinement

Two-dimensional (2D) ultrathin (& SIM;0.5 nm) aluminosilicatebilayer films, consisting of hexagonal prisms (a.k.a. double 6-memberedrings D6R) with acidic bridging hydroxyl groups exposed on the surface,have been previously synthesized on a Ru(0001) surface as a zeolitemodel system. These structures are helpful for mimicking zeolite catalystswith D6R building blocks, such as chabazite. We performed densityfunctional theory calculations to investigate the monomolecular crackingand dehydrogenation of n-butane molecules over theacidic hydroxyl groups of the 2D model system and compared the reactionenergetics with that in bulk chabazite. The intrinsic activation energybarrier is the highest for dehydrogenation and lowest for centralC-C bond cracking in bulk chabazite. The trend of intrinsicenergy barriers for dehydrogenation and terminal and central C-Cbond cracking is reproduced on the 2D aluminosilicate film. Overall,the activation barriers are higher on the 2D film than in bulk chabazitedue to the lack of confinement in the former. We further exploredthe effects of the zeolite channel size on the n-butaneadsorption and monomolecular cracking using different bulk nanoporouszeolite frameworks (TON, MEL, MEI, and VFI). We found that as theconfinement of channels decreases, n-butane adsorptionbecomes weaker, and the intrinsic energy barrier of terminal C-Ccracking increases. The activation energy barriers (dehydrogenationand terminal and central C-C cracking) on the 2D bilayer filmsurface, which may be considered as zeolite cages at the infinitecage size limit, are close to that in VFI with a relatively largechannel size. Comparing the reaction pathway of n-butane terminal C-C cracking in 3D nanocages and on the surfaceof the 2D aluminosilicate film revealed that stabilizing the transitionstates in the 3D nanocages is responsible for the decrease in theintrinsic energy barriers for bulk zeolites.

Automated summary

The cracking reaction of n-butane molecules on a two-dimensional nanomaterial was studied and compared with bulk zeolite. The activation energy barrier for cracking on the nanomaterial was higher than in bulk zeolite due to the lack of confinement. The effects of different zeolite structures on n-butane adsorption and cracking were also explored, revealing that smaller channel sizes led to weaker adsorption and higher activation energy barriers for terminal C-C cracking.