(714b) Assessing the Kinetic Effects of Al Siting on Methanol Dehydration in Different Zeolite Void Environments Using Density Functional Theory

Authors: 
Hoffman, A., University of Florida
Nimlos, C. T., Purdue University
Petro, A., University of Florida
Kester, P. M., Purdue University
Nystrom, S. V. Jr., University of Florida
Gounder, R., Purdue University
Hibbitts, D., University of Florida
Methanol dehydration to dimethyl ether (DME) is a probe reaction that can separately assess effects of acid strength and noncovalent interactions in zeolite voids [1,2]. Methanol dehydration can occur by either a two-step sequential (Steps S1 and S2) or a single-step concerted route (Step C1). Turnover rates for methanol dehydration (per H+) increase as voids approach the size of the kinetically relevant concerted transition state [1]. We assess every T-site in seven zeolite frameworks (MFI, CHA, BEA, FER, AEI, MOR, and ERI) using DFT-calculated methanol dehydration barriers to determine the void sizes and shapes that increase turnover rates to inform zeolite synthesis aiming to optimize environments around active sites. Electronic energy barriers relative to one adsorbed methanol (ΔES1҂) indicate that S1 transition states are stabilized in eight-membered rings (8-MR), particularly in that adjacent to T2 in AEI (ΔES1҂ =123 kJ mol−1). Step C1 electronic barriers relative to one adsorbed methanol (ΔEC1҂) indicate that these larger transition states are stabilized better by 10-MR than 8- or 12-MR. Specifically, the lowest barriers were found on T1 of FER (ΔEC1҂ = 17 kJ mol−1) and T3 of MFI (ΔEC1҂ = 18 kJ mol−1). ΔEC1҂ on MFI range from 18 to 43 kJ mol−1 on T3 and T10, respectively, reflecting the diverse environments in one framework despite the similarly sized straight and sinusoidal 10-MR confining these transition states [3]. These methanol dehydration barriers are governed simultaneously by dispersive forces and H-bonding with framework O atoms; transition states prefer voids that are of a similar size and whose geometries promote H-bonding.

References

[1] Jones, A. J.; Iglesia, E., J. Phys. Chem. C, 2014, 118, 17787–17800

[2] Carr, R. T.; Neurock, M.; Iglesia, E., J. Catal., 2011, 278, 78–93

[3] Ghorbanpour, A.; Rimer, J. D.; Grabow, L. C., ACS Catal., 2016, 6, 2287–2298

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