Shape-selective diffusion of olefins in 8-ring solid acid microporous zeolites

[EN] The diffusion of olefins through 8-ring solid acid microporous zeolites is investigated using molecular dynamics simulations techniques and using a newly developed flexible force field. Within the context of the methanol-to-olefin (MTO) process and the observed product distribution, knowledge of the diffusion paths is essential to obtain molecular level control over the process conditions. Eight-ring zeotype materials are favorably used for the MTO process as they give a selective product distribution toward low-carbon olefins. To investigate how composition, acidity, and flexibility influence the diffusion paths of ethene and propene, a series of isostructural aluminosilicates (zeolites) and silicoaluminophosphates (AlPOs and SAPOs) are investigated with and without randomly distributed acidic sites. Distinct variations in diffusion of ethene are observed in terms of temperature, composition, acidity, and topology (AEI, CHA, AFX). In general, diffusion of ethene is an activated process for which free energy barriers for individual rings may be determined. We observe ring-dependent diffusion behavior which cannot be described solely in terms of the composition and topology of the rings. A new descriptor had to be introduced, namely, the accessible window area (AWA), inspired by implicit solvation models of proteins and small molecules. The AWA may be determined throughout the molecular dynamics trajectories and correlates well with the number of ring crossings at the molecular level and the free energy barriers for ring crossings from one cage to the other. The overall observed diffusivity is determined by molecular characteristics of individual rings for which AWA is a proper descriptor. Temperature-induced changes in framework dynamics and diffusivity may be captured by following the new descriptor throughout the simulations.The computational resources and services used were provided by Ghent University (Stevin Supercomputer Infrastructure). Funding was received from the Research Board of Ghent University (BOF), the Foundation of Scientific Research-Flanders (FWO), and BEL-SPO in the frame of IAP/7/05. V.V.S acknowledges funding from the European Research Council under the European Community's Seventh Framework Programme (FP7(2007-2013) ERC Grant Agreement 240483), and from the European Union's Horizon 2020 research and innovation programme (Consolidator ERC Grant Agreement 647755 - DYNPOR (2015-2020)). G.S. thanks the Spanish government for the provision of Severo Ochoa project (SEV 2012-0267) and SGAI-CSIC for computing time.Ghysels, A.; Moors, S.; Hemelsoet, K.; De Wispelaere, K.; Waroquier, M.; Sastre Navarro, GI.; Van Speybroeck, V. (2015). Shape-selective diffusion of olefins in 8-ring solid acid microporous zeolites. Journal of Physical Chemistry C. 119(41):23721-23734. https://doi.org/10.1021/acs.jpcc.5b06010S23721237341194

Trifluoromethylated Proline Surrogates as Part of "Pro-Pro" Turn-Inducing Templates

Proline is often found as a turn inducer in peptide or protein domains. Exploitation of its restricted conformational freedom led to the development of the d-Pro-l-Pro (corresponding to (R)-Pro-(S)-Pro) segment as a "templating" unit, frequently used in the design of beta-hairpin peptidomimetics, in which conformational stability is, however, inherently linked to the cis-trans isomerization of the prolyl amide bonds. In this context, the stereoelectronic properties of the CF3 group can aid in conformational control. Herein, the impact of alpha-trifluoromethylated proline analogues is examined for the design of enhanced beta-turn inducers. A theoretical conformational study permitted the dipeptide (R)-Pro-(R)-TfmOxa (TfmOxa: 2-trifluoromethyloxazolidine-2-carboxylic acid) to be selected as a template with an increased trans-cis rotational energy barrier. NMR spectroscopic analysis of the Ac-(R)-Pro-(R)-TfmOxa-(S)-Val-OtBu beta-turn model, obtained through an original synthetic pathway, validated the prevalence of a major trans-trans conformer and indicated the presence of an internal hydrogen bond. Altogether, it was shown that the (R)-Pro-(R)-TfmOxa template fulfilled all crucial beta-turn-inducer criteria