Ab initio predictions for elementary adsorption and reaction steps in catalysis by acidic zeolites Joachim Sauer , Fabian Berger, Marcin Rybicki Humboldt University Berlin, Germany Experimental kinetic studies deal with complex reaction networks, whereas computational studies can provide energy barriers, reaction energies and pre-exponentials for individual reaction and adsorption steps. If made with sufficient, i.e. chemical, accuracy, computational predictions are very useful to check critical steps in microkinetic models. Our hybrid high level QM:low level QM method has been shown to yield chemical accuracy (4 kJ/mol for energies and one order of magnitude for rate constants) for the methylation of ethene, propene and butene in H-ZSM-5. We demonstrate the importance of accurate predictions for the heat of adsorption and intrinsic barriers for understanding hydrocarbon activation by acidic zeolites, in particular proton exchange and cracking of alkanes. For alkane cracking in H-MFI, the predicted heats of adsorption at reaction temperatures (650 K) support experimental results derived from spectroscopic measurements [1]. The observed decrease of experimental apparent barriers from propane to pentane for alkane cracking in MFI (28 kJ/mol) is largely due to increasing adsorption strengths (21 kJ/mol) and to a much smaller extent to decreasing intrinsic barriers (7kJ/mol). References 1. Li, H.; Kadam, S. A.; Vimont, A.; Wormsbecher, R. F.; Travert, A. ACS Catal. 2016, 6 , 4536-4548.
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