(606c) Ab-Initio modeling of Site Interconversion and Microkinetic Modeling of Lewis Acid Zeolites for Butadiene Synthesis

The structures of active sites in microporous Lewis acid zeolites are investigated using first principles DFT and microkinetic modeling to quantify the mechanism and most abundant surface intermediates for elementary reactions to form butadiene. Microporous solid acids such as Lewis acid zeolites are an attractive class of catalysts for highly selective oxygenate chemistries^1,2; however, the ability to traverse the full design space in both active metal heteroatom identity and framework topology is limited, because of the synthetic difficulties in preparing a broad range of frameworks substituted with a given heteroatom. To more efficiently establish design rules and structure-function relationships for Lewis acid zeolite materials, a combined computational and experimental approach is therefore useful. Such an approach is most straightforwardly implemented for reactions occurring in the gas phase, such as butadiene synthesis and ethanol dehydration, which allow for close collaboration between theory and experiment by avoiding complications introduced by solvent effects. Ethanol dehydration is therefore considered as a probe reaction to address the kinetic implications of the interconversion of hydrolyzed-open³ and closed configurations of tin heteroatoms in the zeolite Beta topology. Insights gained from the microkinetic model, including adsorbate structures and entropies as well as site speciation, are then applied to the more chemically complex synthesis of butadiene.

The mechanism for butadiene formation is discussed, and partition function-based entropies are used for adsorbed intermediates, which introduce anharmonic degrees of freedom. The formation of diethyl ether, ethene, and crotyl alcohol are considered as the dominant side reactions impacting selectivity to butadiene. Scaling relationships are used to assess the selectivity to butadiene based on heteroatom choice and framework topology, using design principles based on local curvature and van der Waals interactions with the surrounding pore environment. The broader extension to other oxygenate chemistries is discussed and helps quantify the scope of heteroatom and confining pore interactions for designing Lewis acid zeolite catalysts.

References:

(1) Harris, J.W., et al. Catal. 335, 141-154 (2016).

(2) Luo, H.Y., et al. Catal. 320, 198-207 (2014).

(3) Boronat, M., et al. Catal. 234, 111-118 (2005).