Atomistic understanding of heterogeneous catalysis requires knowledge about the active sites of the catalyst and the elementary steps of the catalytic conversion. In zeolites, the catalytic activity arises either from transition metal ions or Brønsted sites. The location of the active sites in the aluminosilicate framework structure is coupled to the position of Al, which is difficult to determine by deiffraction methods, but some progress has recently been made using NMR. Quantum chemical calculations for different framework positions, e.g. for Cu ions, can help to reveal reactivity differences that arise from the distribution of active sites in the framework.<br/>The preparation of thin film model zeolites that are studied with surface science techniques allow a direct characterization of active sites and also inferences on surface curvature effects.<br/>Ab initio calculations that yield free energies for elementary steps with chemical accuracy (4 kJ/mol) allows examining the interplay of local acidity of a site and surface curvature which is key to understanding the shape selective properties of these porous material. For proton exchange reactions the conclusion is reached that variations of energy barriers across different frameworks and different Al positions are determined by heats of adsorption whereas the intrinsic barriers show little variation.<br/>We also provide an accurate value for the proton exchange barrier of isobutane on H-ZSM-5 which is crucial in the discussion of whether alkane activation happens directly via carbonium type transition structures or indirectly via hydride transfer involving alkoxides or carbenium ions.

Ab initio quantum chemistry for understanding active sites in zeolites

255th National Meeting (2018)

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